code: func (i *ImageService) imageIsDangling(imgID image.ID) bool {
  	return !(len(i.referenceStore.References(imgID.Digest())) > 0 || len(i.imageStore.Children(imgID)) > 0)
  }
docs: // imageIsDangling returns whether the given image is "dangling" which means
  // that there are no repository references to the given image and it has no
  // child images.