Add configurable entrypoint to py3_image

[dbruggner]

This makes entrypoint for py3_image configurable so that a user can provide path to a hermetic python environment.

PR Checklist

Please check if your PR fulfills the following requirements:

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

Issue Number: N/A

Currently the Python binary is fixed as /usr/bin/python.

What is the new behavior?

The Python binary can be choses by overrideing the entrypoint for example to point to a hermetic Python.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information