Make Dockerfile more production focused
Make Dockerfile more production focused

I started editing this file to remove the REDIS_HOST env var as we are
removing that from the GOV.UK stack and then decided I should update
this Dockerfile similar to how we've configured static [1], since this
doesn't have all dev and test dependencies installed.

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

alphagov/asset-manager,alphagov/asset-manager,alphagov/asset-manager