[API] Configuration option to set advertised hostname
In development mode, the default root_url of rails is 'localhost'. This gets advertised in the discovery document. Unfortunately, this is a problem when running jobs in Docker, because 'localhost' means the docker container's namespace and not that of the host. This is a problem any time that Rail's default logic for generating the hostname in root_url does not match up with the hostname that actually should be advertised.
API server needs a configuration option to explicitly set the hostname that is advertised by rails.