Mount entire root_project as volume in app service to persist generat…

[DominikCywinski]

Changes

• Mount the whole project (.:/root_project) in the app service with an
  anonymous volume for /root_project/.venv, consistent with the
  celery-worker, celery-beat and flower services. The previous
  ./app and config/.env mounts are covered by the project mount.
• Add migrations/versions/.gitkeep so the folder is tracked in the
  generated project's repo - without it, cloning the project before the
  first migration exists would leave versions/ missing on the host and
  alembic revision would fail in the container (the bind mount shadows
  the image contents).

Closes #90