ALLOWED_HOSTS in a docker container

The [documentation](https://github.com/nexB/scancode.io/blob/main/docs/run-docker.rst#run-the-image) asks to populate `ALLOWED_HOSTS` in `.env` during the _Run_ phase. This will have no effect inside the docker container. `ALLOWED_HOSTS` should be either
1) Placed before building the dockerfile so that `.env` gets copied inside the container and carries the `ALLOWED_HOSTS` value or
2) Be used inside `docker.env`.

2 appears to be a better approach in my opinion.