Updates to fastapi-postgres + various readmes

[commit111]

Details:

• Starter sample now includes defang config set API_KEY command in its README (see bullmq-bullboard-redis README as an example on what it looks like for a list of values)

• These samples have been updated to match starter sample's README:

  • bullmq-bullboard-redis
  • django-postgres
  • fastapi-postgres

• fastapi_postgres has a better compose.yaml and compose.dev.yaml structure

Samples Checklist

✅ All good!

[commit111]

Closed, decided to break it up into multiple pull requests: #263, #264, #265, #266, #267, #268