docker/daemon/daemon_src.go
func newImageSource(ctx context.Context, sys *types.SystemContext, ref daemonReference) (types.ImageSource, error) {
	c, err := newDockerClient(sys)
	if err != nil {
		return nil, errors.Wrap(err, "Error initializing docker engine client")
	}
	// Per NewReference(), ref.StringWithinTransport() is either an image ID (config digest), or a !reference.NameOnly() reference.
	// Either way ImageSave should create a tarball with exactly one image.
	inputStream, err := c.ImageSave(ctx, []string{ref.StringWithinTransport()})
	if err != nil {
		return nil, errors.Wrap(err, "Error loading image from docker engine")
	}
	defer inputStream.Close()

	src, err := tarfile.NewSourceFromStream(inputStream)
	if err != nil {
		return nil, err
	}
	return &daemonImageSource{
		ref:    ref,
		Source: src,
	}, nil
}
func newImageSource(ctx context.Context, sys *types.SystemContext, ref daemonReference) (types.ImageSource, error) {
	c, err := newDockerClient(sys)
	if err != nil {
		return nil, errors.Wrap(err, "Error initializing docker engine client")
	}
	// Per NewReference(), ref.StringWithinTransport() is either an image ID (config digest), or a !reference.NameOnly() reference.
	// Either way ImageSave should create a tarball with exactly one image.
	inputStream, err := c.ImageSave(ctx, []string{ref.StringWithinTransport()})
	if err != nil {
		return nil, errors.Wrap(err, "Error loading image from docker engine")
	}
	defer inputStream.Close()

	src, err := tarfile.NewSourceFromStream(inputStream)
	if err != nil {
		return nil, err
	}
	return &daemonImageSource{
		ref:    ref,
		Source: src,
	}, nil
}
// newImageSource returns a types.ImageSource for the specified image reference.
// The caller must call .Close() on the returned ImageSource.
//
// It would be great if we were able to stream the input tar as it is being
// sent; but Docker sends the top-level manifest, which determines which paths
// to look for, at the end, so in we will need to seek back and re-read, several times.
// (We could, perhaps, expect an exact sequence, assume that the first plaintext file
// is the config, and that the following len(RootFS) files are the layers, but that feels
// way too brittle.)