Automatically create the Docker network "dbs"

[seanh]

I keep running into this error whenever running make services:

Error response from daemon: network dbs not found

It's caused by this networks stuff that was added to h's docker-compose.yml:

https://github.com/hypothesis/h/blob/7cb19f4e08100641d316092a3f25f741a2c78dc2/docker-compose.yml#L29-L34

There was once a command added to h's Makefile to auto-create this network:

https://github.com/hypothesis/h/pull/7673

But it got removed by:

https://github.com/hypothesis/h/commit/abbe35ccbb6c47c1038aaf9b15c304419c30b8f1.

Add the command back to get rid of the network dbs not found errors.

We'll probably have to add support for this to our cookiecutter but h doesn't use the cookiecutter yet so for now this is fine.