code: func (i *ImageService) ExportImage(names []string, outStream io.Writer) error {
  	imageExporter := tarexport.NewTarExporter(i.imageStore, i.layerStores, i.referenceStore, i)
  	return imageExporter.Save(names, outStream)
  }
docs: // ExportImage exports a list of images to the given output stream. The
  // exported images are archived into a tar when written to the output
  // stream. All images with the given tag and all versions containing
  // the same tag are exported. names is the set of tags to export, and
  // outStream is the writer which the images are written to.