Automatically create the Docker network "dbs"

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:

#7673

But it got removed by:

abbe35c.

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.

Makefile

@@ -31,6 +31,7 @@ help:
 .PHONY: services
 services: args?=up -d --wait
 services: python
+	@docker network create dbs 2>/dev/null || true
 	@docker compose $(args)
 
 .PHONY: db