code: func pull(image string) error {
  	var stdout, stderr bytes.Buffer
  	cmd := exec.Command("docker", "pull", image)
  	cmd.Stdout = &stdout
  	cmd.Stderr = &stderr
  	err := cmd.Run()
  	out := stdout.String()
  	// TODO(mpl): if it turns out docker respects conventions and the
  	// "Authentication is required" message does come from stderr, then quit
  	// checking stdout.
  	if err != nil || stderr.Len() != 0 || strings.Contains(out, "Authentication is required") {
  		return fmt.Errorf("docker pull failed: stdout: %s, stderr: %s, err: %v", out, stderr.String(), err)
  	}
  	return nil
  }
docs: // Pull retrieves the docker image with 'docker pull'.