Fix Docker Hub workflow login

[kalebbroo]

Summary

• configure the public Docker Hub username directly in the workflow
• require only the scoped Docker Hub token secret
• fail early with a clear message when the publishing token is absent
• clarify that the first push creates the Docker Hub repository

Verification

• python3 -m unittest discover -s tests -v
• python3 -m py_compile handler.py tests/test_handler.py
• docker build --check .

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between acfa5d0 and 4042a74.

📒 Files selected for processing (2)

• .github/workflows/publish-image.yml
• README.md

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