code: func (cli *Client) ContainerExport(ctx context.Context, containerID string) (io.ReadCloser, error) {
  	serverResp, err := cli.get(ctx, "/containers/"+containerID+"/export", url.Values{}, nil)
  	if err != nil {
  		return nil, err
  	}
  
  	return serverResp.body, nil
  }
docs: // ContainerExport retrieves the raw contents of a container
  // and returns them as an io.ReadCloser. It's up to the caller
  // to close the stream.