Renovate replacement of Docker base image dependency defined in ARGs fails

[tobias-jenkner]

How are you running Renovate?

Self-hosted Renovate

Which platform you running Renovate on?

GitHub.com

Which version of Renovate are you using?

42.42.1

Please tell us more about your question or problem

Renovate fails to create a PR for (replacement) dependency updates in Dockerfiles when the Dockerfile contains ARGs with dependencies:

ARG BASE_IMAGE_NAME=amd64/python
ARG BASE_IMAGE_TAG=3.11

FROM  ${BASE_IMAGE_NAME}:${BASE_IMAGE_TAG}

renovate.json:

{
  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
  "packageRules": [
    {
      "description": "Python replacement",
      "matchDatasources": ["docker"],
      "matchPackageNames": [
        "amd64/python"
      ],
      "replacementName": "arm64v8/python",
      "overridePackageName": "arm64v8/python",
      "overrideDepName": "arm64v8/python"
    }
  ]
}

Renovate does not fail to create a PR for (replacement) dependency updates in Dockerfiles when the Dockerfile does not contain ARGs with dependencies:

FROM  amd64/python:3.11

Minimal reproduction testcase: https://github.com/differenzierbar/renovate-replacement-dockerfile-args

Logs (if relevant)

Logs

DEBUG: depName mismatch (repository=differenzierbar/renovate-replacement-dockerfile-args, baseBranch=main, packageFile=Dockerfile, branch=renovate/arm64v8-python-3.x)
       "manager": "dockerfile",
       "currentDepName": "arm64v8/python",
       "newDepName": "python"
 WARN: Error updating branch: update failure (repository=differenzierbar/renovate-replacement-dockerfile-args, baseBranch=main, branch=renovate/arm64v8-python-3.x)

[tobias-jenkner]

To me this looks like a bug. Can we open an issue?