Support PostgreSQL in official docker image

Could you please install `psycopg2` in the official docker image, or create a variant with it installed? PostgreSQL seems well supported with `DATABASE_ENGINE=transaction_hooks.backends.postgresql_psycopg2` otherwise.

Having to run `apt-get update; apt-get install -y python3-dev libpq-dev build-essential; pip3 install psycopg2; source /var/local/ralph/docker-entrypoint.sh` as the entrypoint seems a little silly... :)