Make API images single-binary

[shaldengeki]

Out of convenience, when creating the API images, we bundle three py_binary target layers together. I think this doubled the size of our images (from ~180 to ~360MB), because we're bundling multiple Python environments.

Instead, we should restructure the image rule to take a single py_library for the API as a parameter. Then create a single py_binary, where:

• We run wait_for_postgres
• We pass the application to run on the CLI as the first argument (CMD)
• We parse the input, and either run migrations, or start the API

That should reduce API container image sizes considerably (hopefully).