code: func GetBuilderImage(docker Docker, config *api.Config) (*PullResult, error) {
  	return pullAndCheck(config.BuilderImage, docker, config.BuilderPullPolicy, config)
  }
docs: // GetBuilderImage processes the config and performs operations necessary to
  // make the Docker image specified as BuilderImage available locally. It
  // returns information about the base image, containing metadata necessary for
  // choosing the right STI build strategy.