Support for generic network

[iBotPeaches]

While setting up a new project, I ran into

➜  docker git:(master) ✗ docker network create xxxx-internal
Error response from daemon: could not find an available, non-overlapping IPv4 address pool among the defaults to assign to the network

Apparently from https://stackoverflow.com/a/53838883/455008, this is because Docker caps out at 30 networks. After that and it refuses to work. I thought about deleting old/legacy projects, but that doesn't help things like Jenkins and onboarding.

I think making an internal network at the dev-ops level and just using that network to pivot between php/nginx could work fine. I don't see any immediate concerns for this plan, so pending merge I can cut another release/tag.

So new projects (instead of making their own network) can just define this network as external and use it.

networks:
  st-internal:
    external:
      name: st-internal