An opinionated Dockerfile linter.
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Latest commit 4bae1ca Jul 27, 2018

README.md

Linter and validator for Dockerfile

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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