consider allowing custom ip subnet in docker network create

this might be helpful for some people, using the production version of `higlass` with the `redis`:

in this line:
https://github.com/hms-dbmi/higlass-docker/blob/41fe22fb517132a3c9c4200a6d4b53dfcf76bf90/start_production.sh#L44

where, some kind of network is created - it uses default ip-subnet which is something like `172.17.0.0/16` - this might interfere with local networking for some people.
Simply adding `--subnet=172.34.0.1/16` or something like `--subnet=$SUBNET` might help to avoid that interference issue.