Cleanup: correct unused code

[gounthar]

Pairing session with @dduportal to remove some unused code.
Replaced NULL with $null in build.ps1 and removed an unused FROM from the Alpine Dockerfile.

Testing done

We verified that the only ALPINE_TAG taken into account is the first one, before the first FROM.

docker buildx bake alpine_jdk17 --load

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[dduportal]

Using auto-merge to ensure cleaned up elements are effect-less