Make Dockerfile more production focused
Make Dockerfile more production focused

This removes REDIS_HOST as GOV.UK no longer use that env var to
configure Redis. The rest of the Dockerfile is then updated in a
consistent manner to static [1], where it's tailored towards production
running - reflecting that no-one can use this for test environments (and
potentially development ones).

[1]: https://github.com/alphagov/static/pull/2327

alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher