Add network option

[leizor]

I was seeking a solution for writing tests that deal with multiple containers that all need to be able to talk to each other.

I found #534 but it looked like it might've gone stale so I decided to build upon the work that they had already started (I hope that's ok!).

Instead of creating the network while starting up containers, I added gnomock.StartNetwork to explicitly add a network. When creating containers, use gnomock.WithNetworkID to tell the container to attach to a created network. After the test is done, use gnomock.StopNetwork to remove the network.

[leizor]

I'm leaving a failing lint:

docker.go:317: Function 'createContainer' is too long (63 > 60) (funlen)
func (d *docker) createContainer(

It seems beyond the scope of this PR to do that refactor but let me know if you'd like me to take a crack at it!