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
Use `isNullOrEmpty(tag)` instead to handle empty strings.