Add support for docker commit method (commit container with changes as new image)

Added to resolve https://github.com/spotify/docker-client/issues/76
This method allows you to use the docker client to commit a new image based on an existing container.
See: https://docs.docker.com/reference/api/docker_remote_api_v1.15/#create-a-new-image-from-a-containers-changes
It's not really clear which are more important, containerId is a must, but repo is not required from what I can see (an empty string commits the image into your local repo if I understand correctly. I don't think it is critical in any case, but if people vote on it, I don't mind changing the order to whatever is prefered by the commitee :smile: