[Feature] add volume mount for `polystore-mongo`

[DavyLandman]

Is your feature request related to a problem? Please describe.
You don't want to restart the container and have lost your updated models.

Describe the solution you'd like
Add a default volume to the polystore-mongo container just like the user database containers get.

Describe alternatives you've considered
Re upload the models everytime after re-up?

Additional context
Currently it generates:

  polystore-mongo:
    image: mongo:latest
    environment:
      MONGO_INITDB_ROOT_USERNAME: admin
      MONGO_INITDB_ROOT_PASSWORD: admin
      MONGO_INITDB_DATABASE: admin
    volumes:
      - ./models/:/docker-entrypoint-initdb.d