code: func (i *ImageService) CommitBuildStep(c backend.CommitConfig) (image.ID, error) {
  	container := i.containers.Get(c.ContainerID)
  	if container == nil {
  		// TODO: use typed error
  		return "", errors.Errorf("container not found: %s", c.ContainerID)
  	}
  	c.ContainerMountLabel = container.MountLabel
  	c.ContainerOS = container.OS
  	c.ParentImageID = string(container.ImageID)
  	return i.CommitImage(c)
  }
docs: // CommitBuildStep is used by the builder to create an image for each step in
  // the build.
  //
  // This method is different from CreateImageFromContainer:
  //   * it doesn't attempt to validate container state
  //   * it doesn't send a commit action to metrics
  //   * it doesn't log a container commit event
  //
  // This is a temporary shim. Should be removed when builder stops using commit.