Added Celery worker docker container

This docker container uses the Blender Cloud image, but a different entry
point. It is not intended to be network-reachable from the outside world.
All it needs are connections to the databases (mongo, redis, rabbit).

docker/4_run/celery-worker.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+
+source /install_scripts.sh
+source /manage.sh operations worker -- -C