Sanity checks for docker containers on Rodan startup

It would be useful for there to be manual checks in the Rodan startup scripts (primarily the Rodan start script `/scripts/start` and the Celery start script `scripts/start-celery`, which are copied to `/run` in all docker containers) to have manual checks to ensure, before trying to run anything, that:

- All docker volumes that should be mounted are accessible from within the container
- All ports that should be exposed are exposed and not already bound

When something goes wrong on startup, it should go wrong loudly, and it should be as obvious as possible where the problem is.