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/bash_history

@@ -2,5 +2,8 @@ bash docker-entrypoint.sh
 env | sort
 apache2ctl start
 apache2ctl graceful
+/manage.sh operations worker -- -C
+celery status --broker amqp://guest:guest@rabbit:5672//
+celery events --broker amqp://guest:guest@rabbit:5672//
 tail -n 40 -f /var/log/apache2/access.log
 tail -n 40 -f /var/log/apache2/error.log