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.

Installation

Global installation with npm package manager.

npm install -g dockerfilelint

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

`HEALTHCHECK`

No additional parameters when only parameter is NONE
Options before CMD are valid
Options before CMD have additional arguments

Misc

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

stackriot-labs/dockerfilelint