server: Specify nginx backend using ip rather than localhost

Created by: keegancsmith

This is an issue we have encountered before were docker doesn't correctly resolve localhost, but does resolve 127.0.0.1 correctly.

We also move this bit of config into the sourcegraph controlled config, since this is already an example where we want to be able to mutate this config.

Fixes https://github.com/sourcegraph/sourcegraph/issues/1890