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
I checked and `comment` is indeed optional. In the interest of treating null & empty strings the same, let's remove the `checkNotNull` and instead add a conditional like you did for `author` and `tag` below.