Modularization and cleanups

[zilchms]

The current docker setup is heavyly dependant on having every required config setup before starting vpt for the first time. I would like to introduce the following changes:

1. Make vpt docker container independent of the docker-compose file
   Currently docker-compose is used to inject the entrypoint into the vpt docker container. I would argue, since the entrypoint should not change in any setup, we can hardcode this into the Dockerfile

2. Make validations during container startup and stop the container if something that is required is missing (and explicitly tell the user)
   Right now, if one doesnt have the master.key file the docker container starts, then crashes and restarts again in a very ugly stacktrace. And the message: Errno::EISDIR: Is a directory @io_fread - /vpt/config/master.key
   I think it would be more user-friendly to error out before trying to start the rails application and give the user some meaning-full feedback (and a section in the docs dedicated to the failure)

3. Maybe the same can be done for the things vpt requires to be there (DB / other docker-containers)