feat: add py_image_layer

[thesayyn]

Replaces #349

Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

Add py_image_layer macro for creating py container images.

Test plan

I will add a test in a follow-up.

[alexeagle]

thanks!

[hartikainen]

I hope I'm not adding any unintentional noise here, especially since I'm still very new to Bazel. I was browsing through these Bazel rules and noticed that there might be a bug here. I wonder if these two lines should be something like:

    groups = dict(default_layer_groups.items() + layer_groups.items())
    group_names = default_layer_groups.keys() + groups.keys()