Improve the Docker deployment

[fozcode]

At present the Docker files and docker-compose script don't make it easy to install TLS certs or to override the QIS settings. We're also using an env var for the database password from the days before docker-compose had secrets support. I propose:

• Change the database password to use a Docker secret
• Update the AS Dockerfile, run script, and docker-compose file to mount TLS files from Docker secrets**
• Update the AS Dockerfile, run script, and docker-compose file to allow the local_settings.py file to be overridden**
• Update the README for the AS image
• Update the README for the Postgres image
• Update the running.md guide

** This also allows multiple AS containers to use the same certs and session secret for load balancing