Docker documentation and Docker installation improvements.

[Leopere]

## Feature Request

As outlined in #300 there is some information lacking in the deployment instructions for docker ideally it should be clarified that certain things need added into the pre-launch checklist.

Documentation update for: https://trigger-happy.readthedocs.io/en/latest/docker_guide/index.html

• mention how to modify the ALLOWED_HOSTS to django-th/docker_settings.py
• consider some kind of environment variable grabbing method so that ALLOWED_HOSTS can be defined at run time via a docker-compose.yml defined environment variable.
• consider adding a docker ENTRYPOINT which will help the user run the preflight installation so instructions can be simplified.
• consider utilizing the postgres containers inbuilt options for creating a default user and database that can be established entirely from the docker-compose.yml variables.
• add docker volume definitions so that the user's data doesn't vanish after a container reboot.