Checking your build configuration
Build checks are a feature introduced in Dockerfile 1.8. It lets you validate your build configuration and conduct a series of checks prior to executing your build. Think of it as an advanced form of linting for your Dockerfile and build options, or a dry-run mode for builds.
You can find the list of checks available, and a description of each, in the Build checks reference.
How build checks work
Typically, when you run a build, Docker executes the build steps in your Dockerfile and build options as specified. With build checks, rather than executing the build steps, Docker checks the Dockerfile and options you provide and reports any issues it detects.
Build checks are useful for:
- Validating your Dockerfile and build options before running a build.
- Ensuring that your Dockerfile and build options are up-to-date with the latest best practices.
- Identifying potential issues or anti-patterns in your Dockerfile and build options.
Build with checks
Build checks are supported in Buildx version 0.15.0 and later. Invoking a build runs the checks by default, and displays any violations in the build output. For example, the following command both builds the image and runs the checks:
$ docker build .
[+] Building 3.5s (11/11) FINISHED
...
1 warning found (use --debug to expand):
- Lint Rule 'JSONArgsRecommended': JSON arguments recommended for CMD to prevent unintended behavior related to OS signals (line 7)
In this example, the build ran successfully, but a
JSONArgsRecommended warning
was reported, because CMD instructions should use JSON array syntax.
More verbose output
Check warnings for a regular docker build display a concise message
containing the rule name, the message, and the line number of where in the
Dockerfile the issue originated. If you want to see more detailed information
about the checks, you can use the --debug flag. For example:
$ docker build --debug .
[+] Building 3.5s (11/11) FINISHED
...
1 warning found:
- JSONArgsRecommended: JSON arguments recommended for CMD to prevent unintended behavior related to OS signals (line 4)
JSON arguments recommended for ENTRYPOINT/CMD to prevent unintended behavior related to OS signals
More info: https://docs.docker.com/go/dockerfile/rule/json-args-recommended/
Dockerfile:4
--------------------
2 |
3 | FROM alpine
4 | >>> CMD echo "Hello, world!"
5 |
--------------------
With the --debug flag, the output includes a link to the documentation for
the check, and a snippet of the Dockerfile where the issue was found.
Check a build without building
To run build checks without actually building, you can use the docker build
command as you typically would, but with the addition of the --check flag.
Here's an example:
$ docker build --check .
Instead of executing the build steps, this command only runs the checks and reports any issues it finds. If there are any issues, they will be reported in the output. For example:
[+] Building 1.5s (5/5) FINISHED
=> [internal] connecting to local controller
=> [internal] load build definition from Dockerfile
=> => transferring dockerfile: 253B
=> [internal] load metadata for docker.io/library/node:22
=> [auth] library/node:pull token for registry-1.docker.io
=> [internal] load .dockerignore
=> => transferring context: 50B
JSONArgsRecommended - https://docs.docker.com/go/dockerfile/rule/json-args-recommended/
JSON arguments recommended for ENTRYPOINT/CMD to prevent unintended behavior related to OS signals
Dockerfile:7
--------------------
5 |
6 | COPY index.js .
7 | >>> CMD node index.js
8 |
--------------------This output with --check shows the
verbose message
for the check.
Unlike a regular build, if any violations are reported when using the --check
flag, the command exits with a non-zero status code.
Fail build on check violations
Check violations for builds are reported as warnings, with exit code 0, by
default. You can configure Docker to fail the build when violations are
reported, using a check=error=true directive in your Dockerfile. This will
cause the build to error out when after the build checks are run, before the
actual build gets executed.
| |
Without the # check=error=true directive, this build would complete with an
exit code of 0. However, with the directive, build check violation results in
non-zero exit code:
$ docker build .
[+] Building 1.5s (5/5) FINISHED
...
1 warning found (use --debug to expand):
- JSONArgsRecommended: JSON arguments recommended for CMD to prevent unintended behavior related to OS signals (line 5)
Dockerfile:1
--------------------
1 | >>> # syntax=docker/dockerfile:1
2 | # check=error=true
3 |
--------------------
ERROR: lint violation found for rules: JSONArgsRecommended
$ echo $?
1
You can also set the error directive on the CLI by passing the
BUILDKIT_DOCKERFILE_CHECK build argument:
$ docker build --check --build-arg "BUILDKIT_DOCKERFILE_CHECK=error=true" .
Skip checks
By default, all checks are run when you build an image. If you want to skip
specific checks, you can use the check=skip directive in your Dockerfile.
The skip parameter takes a CSV string of the check IDs you want to skip.
For example:
# syntax=docker/dockerfile:1
# check=skip=JSONArgsRecommended,StageNameCasing
FROM alpine AS BASE_STAGE
CMD echo "Hello, world!"Building this Dockerfile results in no check violations.
You can also skip checks by passing the BUILDKIT_DOCKERFILE_CHECK build
argument with a CSV string of check IDs you want to skip. For example:
$ docker build --check --build-arg "BUILDKIT_DOCKERFILE_CHECK=skip=JSONArgsRecommended,StageNameCasing" .
Combine error and skip parameters for check directives
To both skip specific checks and error on check violations, pass both the
skip and error parameters separated by a semi-colon (;) to the check
directive in your Dockerfile or in a build argument. For example:
# syntax=docker/dockerfile:1
# check=skip=JSONArgsRecommended,StageNameCasing;error=true$ docker build --check --build-arg "BUILDKIT_DOCKERFILE_CHECK=skip=JSONArgsRecommended,StageNameCasing;error=true" .