InvalidDefaultArgInFrom: Default value for ARG perl:${BASE} results in empty or invalid base image name

[szabgab]

In the log of the CI jobs I see this warning. Any idea why and what is its impact?
For example in this report: https://github.com/Perl/docker-perl-tester/actions/runs/25497149434/job/74819867374

[szabgab]

This is what copilot told me:

This warning appears because of the pattern in the Dockerfile:

ARG BASE
FROM perl:${BASE}

If BASE is not given a default (ARG BASE=...) and not provided at build time, its value is empty during Dockerfile parsing and the FROM statement becomes invalid (effectively perl:).
Impact

If the CI/build always provides BASE, the build works and the warning is mostly a lint / robustness warning.
If BASE is ever missing (local build, matrix error, etc.), the build can fail early because the base image reference is invalid.

How to fix / eliminate the warning

Give BASE a default, e.g.:

ARG BASE=latest
FROM perl:${BASE}

(Or pick a specific default tag/version.) This makes the Dockerfile valid even without build args and silences InvalidDefaultArgInFrom.

If you want, I can also check the workflow to confirm where/how BASE is set in CI.

[atoomic]

Setting a default value for BASE is ok, but it s going to hide the real issue

As we can see we pass INPUT_BUILDARGS: BASE=5.42-slim-bookworm,CPANOUTDATED=1
so the real question is why BASE=5.42-slim-bookworm is not passed

Run $GITHUB_ACTION_PATH/entrypoint.sh
  $GITHUB_ACTION_PATH/entrypoint.sh
  shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
  env:
    INPUT_NAME: perldocker/perl-tester
    INPUT_USERNAME: ***
    INPUT_PASSWORD: ***
    INPUT_REGISTRY: 
    INPUT_SNAPSHOT: 
    INPUT_DEFAULT_BRANCH: 
    INPUT_DOCKERFILE: Dockerfile
    INPUT_WORKDIR: 
    INPUT_CONTEXT: 
    INPUT_BUILDARGS: BASE=5.42-slim-bookworm,CPANOUTDATED=1
    INPUT_BUILDOPTIONS: 
    INPUT_CACHE: 
    INPUT_TAGS: 5.42-slim-bookworm,5.42
    INPUT_TAG_NAMES: 
    INPUT_TAG_SEMVER: 
    INPUT_NO_PUSH: false
    INPUT_PLATFORMS: 


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Login Succeeded
#0 building with "default" instance using docker driver

#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 2.20kB done
#1 WARN: InvalidDefaultArgInFrom: Default value for ARG perl:${BASE} results in empty or invalid base image name (line 3)
#1 DONE 0.0s

#2 [auth] library/perl:pull token for registry-1.docker.io
#2 DONE 0.0s

#3 [internal] load metadata for docker.io/library/perl:5.42-slim-bookworm
#3 DONE 0.4s