Change dependency from `github.com/fsouza/go-dockerclient` to `github.com/docker/docker/client`

We currently vendor fsouza/go-dockerclient to communicate with docker, and would like to change to the official client library instead. This should improve handling Docker containers throughout the codebase.

Docker recently deprecated engine-api/client in favour of docker/client but haven't made a stable release containing that code yet.