hg-inner-http: expose Gunicorn options in general configuration
In !5 (merged) we just kept the configurability by the
GUNICORN_CMD_ARGS environment variable that Heptapod Docker was formerly using.
I'm not 100% sure it will work, i.e., if the environment variable would still be passed down from the Docker container definition (e.g,
docker-compose.yml and other mechanisms).
In any case, it'd be better to support configuring those parameters through