Build container image `hello:latest` using build action

[espenhgn]

Changes proposed in this pull request:

• push hello container image to GitHub container registry from main branch.

Before submitting

• [x] I've read and followed all steps in the Making a pull request section of the CONTRIBUTING docs.
• [ ] I've updated or added any relevant documentation and docstrings where appropriate in the codebase.
• [ ] If this PR fixes a bug, I've added a test that will fail without my fix.
• [ ] If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.