Investigate migrating to official docker client SDK

Nomad was written in the before times, when [fsouza/go-dockerclient](https://github.com/fsouza/go-dockerclient) was the best option for a Go docker client. Since then docker has published [official SDKs](https://docs.docker.com/engine/api/sdk/) including for Go.

Investigate what it would take to migrate, and if it's possible, maybe make it happen