Create a Docker network per workspace

[ishitatsuyuki]

For two reasons:

• We need to isolate container networking while allowing the main proxy to reach them.
• In future we may need docker-compose.

Note that the default Docker configurations only allows 31 networks, and we need to tweak the subnet range to have more.

[jankeromnes]

@ishitatsuyuki Good idea, thanks for looking into this!

Also, I'd love to somehow mesh our hosting servers, e.g. by having every container's icecc talk to all nearby cluster hosts. If that's reasonably doable in a safe manner, it would allow utilizing the cluster's overall aggregate power instead of tying down each server individually with builds.