code: func (r *RootFS) Clone() *RootFS {
  	newRoot := NewRootFS()
  	newRoot.Type = r.Type
  	newRoot.DiffIDs = make([]layer.DiffID, len(r.DiffIDs))
  	copy(newRoot.DiffIDs, r.DiffIDs)
  	return newRoot
  }
docs: // Clone returns a copy of the RootFS