Move `RAILS_ENV` to release image; build discourse_dev from slim (#725)

Move `RAILS_ENV` to release image; build discourse_dev from slim (#725)

Unfortunately there is no way to 'unset' an ENV in a Dockerfile. We were working around this by setting `RAILS_ENV` to an empty string in the discourse_dev dockerfile, but that didn't work in every situation.

The discourse_dev image doesn't rely on anything in the 'release' layer of the base image. In fact, it deletes the entire contents of `/var/www/*`.

This commit resolves the situation by:

- Moving the `RAILS_ENV=production` line to the 'release' layer of the base image
- Updating the discourse_dev image to be based on the 'slim' base image