tox-tests: add PostgreSQL support

Update the tox test runner to also spin up a PostgreSQL service and pass credentials to the tox container. To facilitate testing Django apps, the credentials are passed both in DJANGO_DB_... environment variables and in POSTGRES_... environment variables.

To better match our standard project layout, also load environment variables from compose/base.env if it is present.

You can see this in action in https://gitlab.developers.cam.ac.uk/uis/devops/research-dashboard/webapp/pipelines/700