Get the image id of the local docker layer with the passed tag
    :param docker_tag: docker tag
    :return: Image id as string or None if tag does not exist

def _get_local_image_id(docker_binary, docker_tag):
    """
    Get the image id of the local docker layer with the passed tag
    :param docker_tag: docker tag
    :return: Image id as string or None if tag does not exist
    """
    cmd = [docker_binary, "images", "-q", docker_tag]
    image_id_b = check_output(cmd)
    image_id = image_id_b.decode('utf-8').strip()
    if not image_id:
        raise RuntimeError('Unable to find docker image id matching with tag {}'.format(docker_tag))
    return image_id