Linter and validator for Dockerfile

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

Testing

Start unit tests with npm test

Running

From the command line:

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

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