code: func (buh *blobUploadHandler) createBlobMountOption(fromRepo, mountDigest string) (distribution.BlobCreateOption, error) {
  	dgst, err := digest.Parse(mountDigest)
  	if err != nil {
  		return nil, err
  	}
  
  	ref, err := reference.WithName(fromRepo)
  	if err != nil {
  		return nil, err
  	}
  
  	canonical, err := reference.WithDigest(ref, dgst)
  	if err != nil {
  		return nil, err
  	}
  
  	return storage.WithMountFrom(canonical), nil
  }
docs: // mountBlob attempts to mount a blob from another repository by its digest. If
  // successful, the blob is linked into the blob store and 201 Created is
  // returned with the canonical url of the blob.