Linter and validator for Dockerfile

Dockerfilelint is an node module that analyzes a Dockerfile and looks for common traps, mistakes and helps enforce best practices:

Testing

Start unit tests with npm test, yarn run test, or docker-compose -f docker-compose.test.yml up

Running

From the command line:

./bin/dockerfilelint <path/to/Dockerfile>

Command Line options

Usage: dockerfilelint [files | content..] [options]

Options:
  -o, --output   Specify the format to use for output of linting results. Valid values
                 are `json` or `cli` (default).                               [string]
  -j, --json     Output linting results as JSON, equivalent to `-o json`.    [boolean]
  -v, --version  Show version number                                         [boolean]
  -h, --help     Show help                                                   [boolean]

Examples:
  dockerfilelint Dockerfile         Lint a Dockerfile in the current working
                                    directory

  dockerfilelint test/example/* -j  Lint all files in the test/example directory and
                                    output results in JSON

  dockerfilelint 'FROM latest'      Lint the contents given as a string on the
                                    command line

  dockerfilelint < Dockerfile       Lint the contents of Dockerfile via stdin

Configuring

You can configure the linter by creating a .dockerfilelintrc with the following syntax:

rules:
  uppercase_commands: off

The keys for the rules can be any file in the /lib/reference.js file. At this time, it's only possible to disable rules. They are all enabled by default.

The following rules are supported:

required_params
uppercase_commands
from_first
invalid_line
sudo_usage
apt-get_missing_param
apt-get_recommends
apt-get-upgrade
apt-get-dist-upgrade
apt-get-update_require_install
apkadd-missing_nocache_or_updaterm
apkadd-missing-virtual
invalid_port
invalid_command
expose_host_port
label_invalid
missing_tag
latest_tag
extra_args
missing_args
add_src_invalid
add_dest_invalid
invalid_workdir
invalid_format
apt-get_missing_rm
deprecated_in_1.13

From a Docker container

(Replace the pwd/Dockerfile with the path to your local Dockerfile)

docker run -v `pwd`/Dockerfile:/Dockerfile replicated/dockerfilelint /Dockerfile

Online

If you don't want to install this locally you can try it out on https://fromlatest.io.

Checks performed

`FROM`

This should be the first command in the Dockerfile
Base image should specify a tag
Base image should not use latest tag
Support FROM scratch without a tag
Support the FROM <image>@<digest> syntax
Allow config to specify "allowed" base layers

`MAINTAINER`

Should be followed by exactly 1 parameter (@ sign)

`RUN`

sudo is not included in the command
apt-get [install | upgrade | remove] should include a -y flag
apt-get install commands should include a --no-install-recommends flag
apt-get install commands should be paired with a rm -rf /var/lib/apt/lists/* in the same layer
Avoid running apt-get upgrade or apt-get dist-upgrade
Never run apt-get update without apt-get install on the same line
apk add commands should include a --no-cache flag or be paired with an --update flag with rm -rf /var/cache/apk/* in the same layer
apk add support for --virtual flag
handle best practices for yum operations and cleanup

`CMD`

Only a single CMD layer is allowed
Better handling of escaped quotes
Detect exec format with expected variable substitution

`LABEL`

Format should be key=value

`EXPOSE`

Only the container port should be listed
All ports should be exposed in a single cache layer (line)
The same port number should not be exposed multiple times
Exposed ports should be numeric and in the accepted range

`ENV`

Format of ENV
Best practice of only using a single ENV line to reduce cache layer count

`ADD`

Command should have at least 2 parameters
Source command(s) cannot be absolute or relative paths that exist outside of the current build context
Commands with wildcards or multiple sources require that destination is a directory, not a file
If an ADD command could be a COPY, then COPY is preferred
Using ADD to fetch remote files is discouraged because they cannot be removed from the layer

`COPY`

Implement checking (similar to ADD)
Do not COPY multiple files on a single command to best use cache

`ENTRYPOINT`

Support

`VOLUME`

Format
Any build steps after VOLUME is declare should not change VOLUME contents
If JSON format, double quotes are required

`USER`

Should be followed by exactly 1 parameter

`WORKDIR`

Validate that it has exactly 1 parameter
WORKDIR can only expand variables previously set in ENV commands

`ARG`

Support
Prevent redefining the built in ARGs (proxy)

`ONBUILD`

Support

`STOPSIGNAL`

Validate input
Only present one time

Misc

Only valid Dockerfile commands are present
All commands should have at least 1 parameter
Check that commands are written as upper case commands

johaw/dockerfilelint