code: func GetAssembleUser(docker Docker, config *api.Config) (string, error) {
  	if len(config.AssembleUser) > 0 {
  		return config.AssembleUser, nil
  	}
  	return extractAssembleUser(docker, config.BuilderImage)
  }
docs: // GetAssembleUser finds an assemble user on the given image.
  // This functions receives the config to check if the AssembleUser was defined in command line
  // If the cmd is blank, it tries to fetch the value from the Builder Image defined Label (assemble-user)
  // Otherwise it follows the common flow, using the USER defined in Dockerfile