Update workflow README

[mafirs]

Summary

Update .github/workflows/README.md so the Docker publish workflow documentation matches the current workflow behavior.

This documents that docker-publish.yml runs on push to main, explains that merging a PR creates that main branch push, and clarifies that the test step runs when configured.

Verification

• Compared upstream/main and origin/main
• Created a dedicated PR branch from upstream/main
• Confirmed GitHub compare result: ahead_by: 1, behind_by: 0, files: 1
• Ran:

git diff --check upstream/main..HEAD

Merge note

Do not merge automatically. Merge manually with Create a merge commit only after checks pass and a maintainer confirms.

Rollback

If something goes wrong after merge, use GitHub's Revert action on this PR.

[github-actions]

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