code: func parseImageName(image string) (imageName, error) {
  	l := strings.Split(image, ":")
  	if len(l) != 4 {
  		return imageName{}, fmt.Errorf("image name %q not a valid format", image)
  	}
  	return imageName{l[0], l[1], l[2], l[3]}, nil
  }
docs: // parseImageName parses a publisher:offer:sku:version into those parts.