code: func extractOnBuildUsers(d Docker, imageName string) ([]string, error) {
  	cmds, err := d.GetOnBuild(imageName)
  	var users []string
  	if err != nil {
  		return users, err
  	}
  	for _, line := range cmds {
  		parts := dockerLineDelim.Split(line, 2)
  		if strings.ToLower(parts[0]) != "user" {
  			continue
  		}
  		users = append(users, extractUser(parts[1]))
  	}
  	return users, nil
  }
docs: // extractOnBuildUsers checks a list of Docker ONBUILD instructions for user
  // directives. It returns a list of users specified in the ONBUILD directives.