A minimal docker baseimage to ease creation of X graphical application containers
This is a docker baseimage that can be used to create containers able to run any X application on a headless server very easily. The application's GUI is accessed through a modern web browser (no installation or configuration needed on the client side) or via any VNC client.
Table of Content
- Images
- Getting started
- Using the Baseimage
- Selecting a Baseimage
- Container Startup Sequence
- Container Shutdown Sequence
- Environment Variables
- Ports
- User/Group IDs
- Locales
- Accessing the GUI
- Security
- Initialization Scripts
- Finalization Scripts
- Services
- Configuration Directory
- Adding/Removing Packages
- Container Log
- Log Monitor
- Adding glibc
- Modifying Files With Sed
- Application Icon
- Dark Mode
- Tips and Best Practices
Images
Different docker images are available:
Base Distribution | Docker Image Base Tag | Size |
---|---|---|
Alpine 3.13 | alpine-3.13 | |
Alpine 3.14 | alpine-3.14 | |
Alpine 3.15 | alpine-3.15 | |
Alpine 3.16 | alpine-3.16 | |
Debian 8 | debian-8 | |
Debian 9 | debian-9 | |
Debian 10 | debian-10 | |
Debian 11 | debian-11 | |
Ubuntu 16.04 LTS | ubuntu-16.04 | |
Ubuntu 18.04 LTS | ubuntu-18.04 | |
Ubuntu 20.04 LTS | ubuntu-20.04 |
Content
Here are the main components of the baseimage:
- An init system.
- A process supervisor, with proper PID 1 functionality (proper reaping of processes).
- TigerVNC, a X server with an integrated VNC server.
- JWM, a window manager.
- noVNC, a HTML5 VNC client.
- NGINX, a high-performance HTTP server.
- Useful tools to ease container building.
- Environment to better support dockerized applications.
Versioning
Images are versioned. Version number follows the semantic versioning. The
version format is MAJOR.MINOR.PATCH
, where an increment of the:
MAJOR
version indicates that a backwards-incompatible change has been done.MINOR
version indicates that functionality has been added in a backwards-compatible manner.PATCH
version indicates that a bug fix has been done in a backwards-compatible manner.
Tags
For each distribution-specific image, multiple tags are available:
Tag | Description |
---|---|
distro-vX.Y.Z | Exact version of the image. |
distro-vX.Y | Latest version of a specific minor version of the image. |
distro-vX | Latest version of a specific major version of the image. |
Getting started
The Dockerfile
for your application can be very simple, as only three things
are required:
- Instructions to install the application.
- A script that starts the application (stored at
/startapp.sh
in container). - The name of the application.
Here is an example of a docker file that would be used to run the xterm
terminal.
In Dockerfile
:
# Pull base image.
FROM jlesage/baseimage-gui:alpine-3.15-v4
# Install xterm.
RUN add-pkg xterm
# Copy the start script.
COPY startapp.sh /startapp.sh
# Set the name of the application.
RUN set-cont-env APP_NAME "Xterm"
In startapp.sh
:
#!/bin/sh
exec /usr/bin/xterm
Then, build your docker image:
docker build -t docker-xterm .
And run it:
docker run --rm -p 5800:5800 -p 5900:5900 docker-xterm
You should be able to access the xterm GUI by opening in a web browser:
http://[HOST IP ADDR]:5800
Using the Baseimage
Selecting a Baseimage
Using a baseimage based on Alpine Linux is the recommended choice. Not only because of its small size, but also because Alpine Linux is a distribution based on musl and BusyBox that is designed for security, simplicity and resource efficiency.
However, using this baseimage to integrate an application not part of the Alpine's software repository or without its source code available may be harder. This is because Alpine Linux uses musl C standard library instead of GNU C library (glibc) that most applications are built against. Compatibility between these two libraries is very limited.
Integrating glibc binaries often require to add glibc to the image. See the Adding glibc section for more details.
Else, Debian
and Ubuntu
images are well known Linux distributions that
provide great compatibility with existing applications. Debian
images are
smaller than the Ubuntu
ones.
Container Startup Sequence
When the container is starting, the following steps are performed:
- The init process (
/init
) is invoked. - Internal environment variables are loaded from
/etc/cont-env.d
. - Initialization scripts under
/etc/cont-init.d
are executed in alphabetical order. - Control is given to the process supervisor.
- The service group
/etc/services.d/default
is loaded, along with its dependencies. - Services are started, in proper order.
- Container is now fully started.
Container Shutdown Sequence
There are two ways a container can shutdown:
- When the implemented application terminates.
- When Docker performs a shutdown of the container (e.g via the
docker stop
command).
In both cases, the shutdown sequence is:
- All services are terminated, in reverse order.
- If some processes are still alive, a SIGTERM is sent to everyone.
- After 5 seconds, all remaining processes are forcefully terminated via the SIGKILL signal.
- The process supervisor execute the exit script (
/etc/services.d/exit
). - The exit script executes, in alphabetical order, finalization scripts
defined under
/etc/cont-finish.d/
. - Container is full stopped.
Environment Variables
Environment variables are very useful to customize the behavior of the container and its application.
There are two types of environment variables:
-
Public: These variables are targeted to people using the container. They provide a way to configure it. They are declared in the
Dockerfile
, via theENV
instruction. Their value can be set by users during the creation of the container, via the-e "<VAR>=<VALUE>"
argument of thedocker run
command. Also, many Docker container management systems use these variables to automatically provide configuration parameters to the user. -
Internal: These variables are the ones that don't need to be exposed to users. They are useful for the application itself, but are not intended to be changed by users.
NOTE: If a variable is defined as both an internal and public one, the value of the public variable takes precedence.
Public Environment Variables
The following public environment variables are provided by the baseimage:
Variable | Description | Default |
---|---|---|
USER_ID |
ID of the user the application runs as. See User/Group IDs to better understand when this should be set. | 1000 |
GROUP_ID |
ID of the group the application runs as. See User/Group IDs to better understand when this should be set. | 1000 |
SUP_GROUP_IDS |
Comma-separated list of supplementary group IDs of the application. | "" |
UMASK |
Mask that controls how file permissions are set for newly created files. The value of the mask is in octal notation. By default, the default umask value is 0022 , meaning that newly created files are readable by everyone, but only writable by the owner. See the online umask calculator at http://wintelguy.com/umask-calc.pl. |
0022 |
TZ |
TimeZone used by the container. Timezone can also be set by mapping /etc/localtime between the host and the container. |
Etc/UTC |
KEEP_APP_RUNNING |
When set to 1 , the application will be automatically restarted when it crashes or terminates. |
0 |
APP_NICENESS |
Priority at which the application should run. A niceness value of -20 is the highest priority and 19 is the lowest priority. The default niceness value is 0. NOTE: A negative niceness (priority increase) requires additional permissions. In this case, the container should be run with the docker option --cap-add=SYS_NICE . |
0 |
INSTALL_PACKAGES |
Space-separated list of packages to install during the startup of the container. Packages are installed from the repository of the Linux distribution this container is based on. ATTENTION: Container functionality can be affected when installing a package that overrides existing container files (e.g. binaries). | "" |
CONTAINER_DEBUG |
Set to 1 to enable debug logging. |
0 |
DISPLAY_WIDTH |
Width (in pixels) of the application's window. | 1920 |
DISPLAY_HEIGHT |
Height (in pixels) of the application's window. | 1080 |
DARK_MODE |
When set to 1 , dark mode is enabled for the application. |
0 |
SECURE_CONNECTION |
When set to 1 , an encrypted connection is used to access the application's GUI (either via a web browser or VNC client). See the Security section for more details. |
0 |
SECURE_CONNECTION_VNC_METHOD |
Method used to perform the secure VNC connection. Possible values are SSL or TLS . See the Security section for more details. |
SSL |
SECURE_CONNECTION_CERTS_CHECK_INTERVAL |
Interval, in seconds, at which the system verifies if web or VNC certificates have changed. When a change is detected, the affected services are automatically restarted. A value of 0 disables the check. |
60 |
WEB_LISTENING_PORT |
Port used by the web server to serve the UI of the application. This port is used internally by the container and it is usually not required to be changed. By default, a container is created with the default bridge network, meaning that, to be accessible, each internal container port must be mapped to an external port (using the -p or --publish argument). However, if the container is created with another network type, changing the port used by the container might be useful to prevent conflict with other services/containers. NOTE: a value of -1 disables listening, meaning that the application's UI won't be accessible over HTTP/HTTPs. |
5800 |
VNC_LISTENING_PORT |
Port used by the VNC server to serve the UI of the application. This port is used internally by the container and it is usually not required to be changed. By default, a container is created with the default bridge network, meaning that, to be accessible, each internal container port must be mapped to an external port (using the -p or --publish argument). However, if the container is created with another network type, changing the port used by the container might be useful to prevent conflict with other services/containers. NOTE: a value of -1 disables listening, meaning that the application's UI won't be accessible over VNC. |
5900 |
VNC_PASSWORD |
Password needed to connect to the application's GUI. See the VNC Password section for more details. | "" |
ENABLE_CJK_FONT |
When set to 1 , open-source computer font WenQuanYi Zen Hei is installed. This font contains a large range of Chinese/Japanese/Korean characters. |
0 |
Internal Environment Variables
The following internal environment variables are provided by the baseimage:
Variable | Description | Default |
---|---|---|
APP_NAME |
Name of the implemented application. | DockerApp |
APP_VERSION |
Version of the implemented application. | (unset) |
DOCKER_IMAGE_VERSION |
Version of the Docker image that implements the application. | (unset) |
HOME |
Home directory. | "" |
XDG_CONFIG_HOME |
Defines the base directory relative to which user specific configuration files should be stored. | /config/xdg/config |
XDG_DATA_HOME |
Defines the base directory relative to which user specific data files should be stored. | /config/xdg/data |
XDG_CACHE_HOME |
Defines the base directory relative to which user specific non-essential data files should be stored. | /config/xdg/cache |
TAKE_CONFIG_OWNERSHIP |
When set to 0 , ownership of the content of the /config directory is not taken during startup of the container. |
1 |
INSTALL_PACKAGES_INTERNAL |
Space-separated list of packages to install during the startup of the container. Packages are installed from the repository of the Linux distribution this container is based on. | "" |
Adding/Removing Internal Environment Variables
Internal environment variables are defined by adding a file to
/etc/cont-env.d/
inside the container, where the name of the file is the name
of the variable and its value is defined by the content of the file.
If the file has execute permission, the init process will execute the program and the value of the environment variable is expected to be printed to its standard output.
NOTE: If the program exits with the return code 100
, the environment
variable is not set (this is different than being set with an empty
value).
NOTE: Any output to stderr performed by the program is redirected to the container's log.
NOTE: The helper set-cont-env
can be used to set internal environment
variables from the Dockerfile.
Availability
Since public environment variables are defined during the creation of the container, they are always available to all your scripts and services, as soon as the container starts.
For internal environment variables, they first need to be loaded during the startup of the container before they can be used. Since this is done before running init scripts and services, availability should not be an issue.
Docker Secrets
Docker secrets is a functionality available to swarm services that offers a secure way to store sensitive information such as username, passwords, etc.
This baseimage automatically exports, as environment variables, Docker secrets that follow this naming convention:
CONT_ENV_<environment variable name>
For example, for a secret named CONT_ENV_MY_PASSWORD
, the environment variable
MY_PASSWORD
is created, with its content matching the one of the secret.
Ports
Here is the list of ports used by the baseimage. With a container using the
default bridge network, these ports can be mapped to the host via the
-p <HOST_PORT>:<CONTAINER_PORT>
parameter.
Port | Mapping to host | Description |
---|---|---|
5800 | Optional | Port to access the application's GUI via the web interface. Mapping to the host is optional if access through the web interface is not wanted. For a container not using the default bridge network, the port can be changed with the WEB_LISTENING_PORT environment variable. |
5900 | Optional | Port to access the application's GUI via the VNC protocol. Mapping to the host is optional if access through the VNC protocol is not wanted. For a container not using the default bridge network, the port can be changed with the VNC_LISTENING_PORT environment variable. |
User/Group IDs
When mapping data volumes (via the -v
flag of the docker run
command),
permissions issues can occur between the host and the container. Files and
folders of a data volume are owned by a user, which is probably not the same as
the default user under which the implemented application is running. Depending
on permissions, this situation could prevent the container from accessing files
and folders on the shared volume.
To avoid this problem, you can specify the user the application should run as.
This is done by passing the user ID and group ID to the container via the
USER_ID
and GROUP_ID
environment variables.
To find the right IDs to use, issue the following command on the host, with the user owning the data volume on the host:
id <username>
Which gives an output like this one:
uid=1000(myuser) gid=1000(myuser) groups=1000(myuser),4(adm),24(cdrom),27(sudo),46(plugdev),113(lpadmin)
The value of uid
(user ID) and gid
(group ID) are the ones that you should
be given the container.
Locales
The default locale of the container is set to POSIX
. If this cause issues
with your application, the proper locale can be installed. For example, adding
the following instructions to your Dockerfile
set the locale to en_US.UTF-8
.
RUN \
add-pkg locales && \
sed-patch 's/# en_US.UTF-8 UTF-8/en_US.UTF-8 UTF-8/' /etc/locale.gen && \
locale-gen
ENV LANG=en_US.UTF-8
NOTE: Locales are not supported by musl
C standard library on Alpine
.
See:
Accessing the GUI
Assuming that container's ports are mapped to the same host's ports, the graphical interface of the application can be accessed via:
- A web browser:
http://<HOST IP ADDR>:5800
- Any VNC client:
<HOST IP ADDR>:5900
Security
By default, access to the application's GUI is done over an unencrypted connection (HTTP or VNC).
Secure connection can be enabled via the SECURE_CONNECTION
environment
variable. See the Environment Variables section for
more details on how to set an environment variable.
When enabled, application's GUI is performed over an HTTPs connection when accessed with a browser. All HTTP accesses are automatically redirected to HTTPs.
When using a VNC client, the VNC connection is performed over SSL. Note that few VNC clients support this method. SSVNC is one of them.
SSVNC
SSVNC is a VNC viewer that adds encryption security to VNC connections.
While the Linux version of SSVNC works well, the Windows version has some
issues. At the time of writing, the latest version 1.0.30
is not functional,
as a connection fails with the following error:
ReadExact: Socket error while reading
However, for your convienence, an unoffical and working version is provided here:
https://github.com/jlesage/docker-baseimage-gui/raw/master/tools/ssvnc_windows_only-1.0.30-r1.zip
The only difference with the offical package is that the bundled version of
stunnel
has been upgraded to version 5.49
, which fixes the connection
problems.
Certificates
Here are the certificate files needed by the container. By default, when they are missing, self-signed certificates are generated and used. All files are PEM encoded, x509 certificates.
Container Path | Purpose | Content |
---|---|---|
/config/certs/vnc-server.pem |
VNC connection encryption. | VNC server's private key and certificate, bundled with any root and intermediate certificates. |
/config/certs/web-privkey.pem |
HTTPs connection encryption. | Web server's private key. |
/config/certs/web-fullchain.pem |
HTTPs connection encryption. | Web server's certificate, bundled with any root and intermediate certificates. |
NOTE: To prevent any certificate validity warnings/errors from the browser or VNC client, make sure to supply your own valid certificates.
NOTE: Certificate files are monitored and relevant daemons are automatically restarted when changes are detected.
VNC Password
To restrict access to your application, a password can be specified. This can be done via two methods:
- By using the
VNC_PASSWORD
environment variable. - By creating a
.vncpass_clear
file at the root of the/config
volume. This file should contains the password in clear-text. During the container startup, content of the file is obfuscated and moved to.vncpass
.
The level of security provided by the VNC password depends on two things:
- The type of communication channel (encrypted/unencrypted).
- How secure access to the host is.
When using a VNC password, it is highly desirable to enable the secure connection to prevent sending the password in clear over an unencrypted channel.
Access to the host by unexpected users with sufficient privileges can be dangerous as they can retrieve the password with the following methods:
- By looking at the
VNC_PASSWORD
environment variable value via thedocker inspect
command. By defaut, thedocker
command can be run only by the root user. However, it is possible to configure the system to allow thedocker
command to be run by any users part of a specific group. - By decrypting the
/config/.vncpass
file. This requires the user to have the appropriate permission to read the file: it has to be root or be the user defined by theUSER_ID
environment variable.
DH Parameters
Diffie-Hellman (DH) parameters define how the DH key-exchange is performed. More details about this algorithm can be found on the OpenSSL Wiki.
DH Parameters are saved into the PEM encoded file located inside the container
at /config/certs/dhparam.pem
. By default, when this file is missing, 2048
bits DH parameters are automatically generated. Note that this one-time
operation takes some time to perform and increases the startup time of the
container.
Initialization Scripts
During the container startup, initialization scripts are executed in alphabetical order. They are executed before starting services.
Initialization scripts are located at /etc/cont-init.d/
inside the container.
To have a better predictability of the execution order, name of the scripts
follows the XX-name.sh
format, where XX
is a sequence number.
The following ranges are used by the baseimage:
- 10-29
- 70-89
Unless specific needs are required, containers built against this baseimage should use the range 50-59.
Finalization Scripts
Finalization scripts are executed, in alphabetical order, during the shutdown process of the container. They are executed after all services have been stopped.
Finalization scripts are located under /etc/cont-finish.d/
inside the
container.
Services
Services are programs handled by the process supervisor that run in background. When a service dies, it can be configured to be automatically restarted.
Services are defined under /etc/services.d/
in the container. Each service
has its own directory, in which different files are used to store the behavior
of the service.
The content of files provides the value for the associated configuration setting. If the file has execution permission, it will be executed by the process supervisor and its output is taked as the value of the configuration setting.
File | Type | Description | Default |
---|---|---|---|
run | Program | The program to run. | N/A |
is_ready | Program | Program invoked by the process supervisor to verify if the service is ready. The program should exit with an exit code of 0 when service is ready. PID of the service if given to the program as parameter. |
N/A |
kill | Program | Program to run when service needs to be killed. The PID of the service if given to the program as parameter. Note that the TERM signal is still sent to the service after executing the program. |
N/A |
finish | Program | Program invoked when the service terminates. The service's exit code is given to the program as parameter. | N/A |
params | String | Parameter for the service's program to run. One parameter per line. | No parameter |
environment | String | Environment to use for the service. One environment variable per line, of the form key=value . |
Environment untouched |
respawn | Boolean | Whether or not the process must be respawned when it dies. | FALSE |
sync | Boolean | Whether or not the process supervisor waits until the service ends. This is mutually exclusive with respawn . |
FALSE |
ready_timeout | Unsigned integer | Maximum amount of time (in milliseconds) to wait for the service to be ready. | 5000 |
interval | Interval | Interval, in seconds, at which the service should be executed. This is mutually exclusive with respawn . |
No interval |
uid | Unsigned integer | The user ID under which the service will run. | $USER_ID |
gid | Unsigned integer | The group ID under which the service will run. | $GROUP_ID |
sgid | Unsigned integer | List of supplementary group IDs of the service. One group ID per line. | Empty list |
umask | Octal integer | The umask value (in octal notation) of the service. | 0022 |
priority | Signed integer | Priority at which the service should run. A niceness value of -20 is the highest priority and 19 is the lowest priority. | 0 |
workdir | String | The working directory of the service. | Service's directory path |
ignore_failure | Boolean | When set, the inability to start the service won't prevent the container to start. | FALSE |
shutdown_on_terminate | Boolean | Indicates that the container should be shut down when the service terminates. | FALSE |
min_running_time | Unsigned integer | The minimum amount of time (in milliseconds) the service should be running before considering it as ready. | 500 |
disabled | Boolean | Indicates that the service is disabled, meaning that it won't be loaded nor started. | FALSE |
.dep | Boolean | Indicates that the service depends on another one. For example, having srvB.dep means that srvB should be started before this service. |
N/A |
The following table provides more details about some value types:
Type | Description |
---|---|
Program | An executable binary, a script or a symbolic link to the program to run. The program file must have the execute permission. |
Boolean | A boolean value. A true value can be 1 , true , on , yes , enable , enabled . A false value can be 0 , false , off , no , disable , disabled . Values are case insensitive. Also, the presence of an empty file indicates a true value (i.e. the file can be "touched"). |
Interval | An unsigned integer value. The following values are also accepted (case insensitive): yearly , monthly , weekly , daily , hourly . |
Service Group
A service group is a service for which there is no run
program. The process
supervisor will only load its dependencies.
Default Service
During startup, the process supervisor first load the service group default
.
This service group contains dependencies to services that should be started
and that are not a dependency of the app
service.
Service Readiness
By default, a service is considered ready once it has been successfully launched and ran for a minimum amount of time (500ms by default).
This behavior can be adjusted with the following methods:
- By adjusting the minimum amount of time the service should run before
considering it as ready. This can be done by adding the
min_running_time
file to the service's directory. - By informing the process supervisor when the service is ready. This is done
by adding the
is_ready
program to the service's directory, along withready_timeout
file to indicate the maximum amount of time to wait for the service to be ready.
Configuration Directory
Applications often need to write configuration, data, states, logs, etc.
Inside the container, this data should be stored under the /config
directory.
This directory is intended to be mapped to a folder on the host. The goal is to write stuff outside the container to keep this data persistent.
NOTE: During the container startup, ownership of this folder and all its
content is taken. This is to make sure that /config
can be accessed
by the user configured through USER_ID
/GROUP_ID
. This behavior
can be adjusted via the TAKE_CONFIG_OWNERSHIP
internal environment
variable.
Application's Data Directories
A lot of applications use the environment variables defined by the
XDG Base Directory Specification to determine where to store
various data. The baseimage sets these variables so they all fall under
/config/
:
- XDG_DATA_HOME=/config/xdg/data
- XDG_CONFIG_HOME=/config/xdg/config
- XDG_CACHE_HOME=/config/xdg/cache
Adding/Removing Packages
To add or remove packages, use the helpers add-pkg
and del-pkg
provided by
this baseimage. To minimize the size of the container, these tools perform
proper cleanup and make sure that no useless files are left after addition or
removal of packages.
Also, these tools can be used to easily install a group packages temporarily.
Using the --virtual NAME
parameter, this allows installing packages and remove
them at a later time using the provided NAME
(no need to repeat given
packages).
Note that if a specified package is already installed, it will be ignored and
will not be removed automatically. For example, the following commands could be
added to Dockerfile
to compile a project:
RUN \
add-pkg --virtual build-dependencies build-base cmake git && \
git clone https://myproject.com/myproject.git
make -C myproject && \
make -C myproject install && \
del-pkg build-dependencies
Supposing that, in the example above, the git
package was already installed
when the call to add-pkg
is performed, running del-pkg build-dependencies
doesn't remove it.
Container Log
Everything written to the standard output and standard error output of scripts
executed by the init process and services is saved into the container's log.
The container log can be viewed with the command
docker logs <name of the container>
.
To ease consultation of the log, all messages are prefixed with the name of the
service or script. Also, it is a good idea to limit the number of information
written to this log. If a program's output is too verbose, it is preferable
to redirect it to a file. For example, the run
command of a service that
redirects the standard output and standard error output to different files
could be:
#!/bin/sh
exec /usr/bin/my_service > /config/log/my_service_out.log 2> /config/log/my_service_err.log
Log Monitor
The baseimage include a simple log monitor. This monitor allows sending notification(s) when a particular message is detected in a log or status file.
This system has two main components: notification definitions and notification backends (targets). Definitions describe properties of a notification (title, message, severity, etc) and how it is triggered (filtering function). Once a matching string is found in a file, a notification is triggered and sent to one or more backends. A backend can implement any functionality. For example, it could send the notification to the container's log, a file or an online service.
Monitored Files
File(s) to be monitored can be set in the configuration file located at
/etc/logmonitor/logmonitor.conf
. There are two settings to look at:
LOG_FILES
: Comma-separated list of absolute paths to log files to be monitored. A log file is a file having new content appended to it.STATUS_FILES
: Comma-separated list of absolute paths to status files to be monitored. A status file doesn't have new content appended. Instead, its whole content is refreshed/overwritten periodically.
Notification Definition
The definition of a notification consists in multiple files, stored in a
directory under /etc/logmonitor/notifications.d
. For example, definition of
notification MYNOTIF
is found under
/etc/logmonitor/notifications.d/MYNOTIF/
.
The following table describe files part of the definition:
File | Mandatory | Description |
---|---|---|
filter |
Yes | Program (script or binary with executable permission) used to filter messages from a log file. It is invoked by the log monitor with a single argument: a line from the log file. On a match, the program should exit with a value of 0 . Any other values is interpreted as non-match. |
title |
Yes | File containing the title of the notification. To produce dynamic content, the file can be a program (script or binary with executable permission). In this case, the program is invoked by the log monitor with the matched message from the log file as the single argument. Output of the program is used as the notification's title. |
desc |
Yes | File containing the description/message of the notification. To produce dynamic content, the file can be a program (script or binary with executable permission). In this case, the program is invoked by the log monitor with the matched message from the log file as the single argument. Output of the program is used as the notification's description/message. |
level |
Yes | File containing severity level of the notification. Valid severity level values are ERROR , WARNING or INFO . To produce dynamic content, the file can be a program (script or binary with executable permission). In this case, the program is invoked by the log monitor with the matched message from the log file as the single argument. Output of the program is used as the notification's severity level. |
Notification Backend
Definition of a notification backend is stored in a directory under
/etc/logmonitor/targets.d
. For example, definition of STDOUT
backend is
found under /etc/logmonitor/notifications.d/STDOUT/
. The following table
describe files part of the definition:
File | Mandatory | Description |
---|---|---|
send |
Yes | Program (script or binary with executable permission) that sends the notification. It is invoked by the log monitor with the following notification properties as arguments: title, description/message and the severity level. |
debouncing |
No | File containing the minimum amount time (in seconds) that must elapse before sending the same notification with this backend. A value of 0 means infinite (notification is sent once). If this file is missing, no debouncing is done. |
NOTE: The log monitor invokes the notification backend and then waits for
its termination. So it is important to make sure that the execution of the
send
program terminates after a reasonable amount of time.
By default, the baseimage contains the following notification backends:
Backend | Description | Debouncing time |
---|---|---|
stdout |
Display a message to the standard output, making it visible in the container's log. Message of the format is {LEVEL}: {TITLE} {MESSAGE} . |
21 600s (6 hours) |
yad |
Display the notification in a window box, visible in the application's GUI. | Infinite |
Adding glibc
For baseimages based on Alpine Linux, glibc can be installed to the image by
adding the following line to your Dockerfile
:
RUN install-glibc
Modifying Files With Sed
sed
is a useful tool often used in container builds to modify files. However,
one downside of this method is that there is no easy way to determine if sed
actually modified the file or not.
It's for this reason that the baseimage includes a helper that gives sed
a
"patch-like" behavior: if applying a sed expression results in no change on the
target file, then an error is reported. This helper is named sed-patch
and
has the following usage:
sed-patch [SED_OPT]... SED_EXPRESSION FILE
Note that the sed option -i
(edit files in place) is already supplied by the
helper.
It can be used in Dockerfile
, for example, like this:
RUN sed-patch 's/Replace this/By this/' /etc/myfile
If running this sed expression doesn't bring any change to /etc/myfiles
, the
command fails and thus, the Docker build also.
Application Icon
A picture of your application can be added to the image. This picture is displayed in the WEB interface's navigation bar. This is also the master picture used to generate favicons that support different browsers and platforms.
Add the following command to your Dockerfile
, with the proper URL pointing to
your master icon: The master icon should be a square PNG image with a size of
at least 260x260 for optimal results.
# Generate and install favicons.
RUN \
APP_ICON_URL=https://github.com/jlesage/docker-templates/raw/master/jlesage/images/generic-app-icon.png && \
install_app_icon.sh "$APP_ICON_URL"
Note that favicons are generated by RealFaviconGenerator.
Dark Mode
Dark mode can be enabled via the DARK_MODE
environment variable. When
enabled, the web interface used to display the application is automatically
adjusted accordingly.
For the application itself, supporting dark mode is more complicated. Applications don't use the same toolkit to build their UI and each toolkit has its own way to activate the dark mode.
The baseimage provides support for the GTK and QT toolkits.
GTK
When dark mode is enabled, the baseimage automatically setup the environment
to force the application to use a dark theme. Under the hood, this is done by
setting the GTK_THEME
environment variable to Adwaita:dark
.
QT
When dark mode is enabled, the baseimage automatically setup the environment
to force the application to use a dark theme. Under the hood, this is done by
setting the QT_STYLE_OVERRIDE
environment variable to Adwaita-Dark
.
In addition, the application's Dockerfile should install the Adwaita
style/theme. It is provided by the adwaita-qt
package, available from the
Ubuntu, Debian or Alpine Linux software repositories.
NOTE: Dark mode is currently supported by QT5 and QT6.
Tips and Best Practices
Do Not Modify Baseimage Content
Try to avoid modifications to files provided by the baseimage. This minimizes the risk of breaking your container after using a new version of the baseimage.
Default Configuration Files
It is often useful to keep the original version of a configuration file. For example, a copy of the original file could be modified by an initialization script before being installed.
These original files, also called default files, should be stored under the
/defaults
directory inside the container.
The $HOME Variable
The application is run under a Linux user having its own ID. This user has no login capability, has no password, no valid login shell and no home directory. It is effectively a kind of user used by daemons.
Thus, by default, the $HOME
environment variable is not set. While this
should be fine in most case, some applications may expect the $HOME
environment variable to be set (since normally the application is run by a
logged user) and may not behave correctly otherwise.
To make the application happy, the home directory can be set at the beginning
of the startapp.sh
script:
export HOME=/config
Adjust the location of the home directory to fit your needs. However, if the
application uses the home directory to write data, make sure it is done in a
volume mapped to the host (e.g. /config
),
Note that the same technique can be used by services, by exporting the home
directory into their run
script.
Referencing Linux User/Group
The Linux user/group under which the application is running can be referenced via:
- Its ID, as indicated by the
USER_ID
/GROUP_ID
environment variable. - By the user/group
app
. Theapp
user/group is setup during the startup to match the configuredUSER_ID
/GROUP_ID
.
rootfs
Directory
Using All files that need to be copied into the container should be stored in your
source tree under the directory rootfs
. The folder structure into this
directory should reflect the structure inside the container. For example, the
file /etc/cont-init.d/my-init.sh
inside the container should be saved as
rootfs/etc/cont-init.d/my-init.sh
in your source tree.
This way, copying all the required files to the correct place into the container
can be done with this single line in your Dockerfile
:
COPY rootfs/ /
Maximizing Only the Main Window
By default, the application's window is maximized and decorations are hidden. When the application has multiple windows, this behavior may need to be restricted to only the main one.
The window manager can be configured to apply different behaviors for different windows of the application. A specific window is identified by matching one or more of its properties:
- Name of the window.
- Class of the window.
- Type of the window.
To find the value of a property for a particular window:
- Create and start an instance of the container.
- From the host, start the
xprop
tool:
docker exec [container name or id] env DISPLAY=:0 xprop
- Access the GUI of the application and click somewhere on the interested window.
- Information about that window will be printed.
The following table shows how to find the relevant information:
Property | Value |
---|---|
Name | The first string of WM_CLASS . |
Class | The second string of WM_CLASS . |
Type | The type of the window is given by _NET_WM_WINDOW_TYPE . Property's value should be translated to one of the following values: desktop , dialog , dock , menu , normal , notification , splash , toolbar , utility . |
Title | The value of WM_NAME . |
By default, the window manager configuration matches only the type of the
window, which must be normal
. If more restrictions are needed, matching the
name of the window can help.
To do this, matching criterias can be defined using the file located at
/etc/jwm/main-window-selection.jwmrc
in the container. This file should have
one matching critera per line, in XML format. For example, to match against
both the type and the name of the window, the file content should be:
<Type>normal</Type>
<Name>My Application</Name>
Note that a regex can be used as a property's value.
See the JWM documentation for more details: https://joewing.net/projects/jwm/config.html
Adaptations from the 3.x Version
For existing applications using the previous version of the baseimage, few adaptations are needed when updating to the new baseimage. Here are a few tips:
- Verify exposed environment variables: each of them should be categorized as a public or private one. See the Environment Variables section.
- Initialization scripts should be renamed to have the proper naming format. See the Initialization Scripts section.
- Parameters/definition of services should be adjusted for the new system. See the Services section.
- Verify that no scripts are using
with-contenv
in their shebang (e.g. from init scripts). - Set the
APP_VERSION
andDOCKER_IMAGE_VERSION
internal environment variables when/if needed. - Any adjustment to the window manager config (e.g. to maximize only the main window) should be readapted to the new window manager. See the Maximizing Only the Main Window section.