Unset ARGs in FROM statement leads to "invalid-name"

[idelchi]

What would you like help with?

I would like help with my configuration

How are you running Renovate?

Self-hosted

If you're self-hosting Renovate, tell us which platform (GitHub, GitLab, etc) and which version of Renovate.

GitLab, v37.440.7

Please tell us more about your question or problem

A Dockerfile with an ARG declaration for the registry source

ARG REGISTRY
FROM ${REGISTRY}python:3.12-slim

leads to REGISTRY not being resolved correctly (see logs)

whereas

ARG REGISTRY=
FROM ${REGISTRY}python:3.12-slim

will correctly scan dependencies from docker.io.

Is there a way to achieve the same behaviour without needing to set the ARG to an empty value?

Logs (if relevant)

Logs

DEBUG: packageFiles with updates (repository=local)
       "config": {
         "dockerfile": [
           {
             "deps": [
               {
                 "skipReason": "invalid-name",
                 "replaceString": "${REGISTRY}python:3.12-slim",
                 "autoReplaceStringTemplate": "{{depName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}",
                 "datasource": "docker",
                 "depType": "final",
                 "updates": []
               }
             ],
             "packageFile": "images\\cloud\\Dockerfile"
           }
         ]
       }

[rarkins]

Try registryAliases

[idelchi]

Thank you @rarkins !