directory/directory_transport.go
func (ref dirReference) NewImageSource(ctx context.Context, sys *types.SystemContext) (types.ImageSource, error) {
	return newImageSource(ref), nil
}
func (ref dirReference) NewImageSource(ctx context.Context, sys *types.SystemContext) (types.ImageSource, error) {
	return newImageSource(ref), nil
}
// NewImageSource returns a types.ImageSource for this reference.
// The caller must call .Close() on the returned ImageSource.