feat: support custom python entrypoints

[DerekTBrown]

PR Checklist

Please check if your PR fulfills the following requirements:

• [X] Tests for the changes have been added (for bug fixes / features)
• [X] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Documentation content changes
• [ ] Other... Please describe:

What is the current behavior?

py3_image always uses /usr/bin/python as an entrypoint, which may or may not be correct for some base images. For instance, many images provide /usr/bin/python3, and don't provide a symlink to /usr/bin/python.

What is the new behavior?

This PR allows a developer to override the entrypoint of a py3_image or a py_image.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[DerekTBrown]

I just found this related thread:

https://github.com/bazelbuild/rules_docker/issues/1247#issuecomment-551262041