Add max concurrent downloads support.

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/server/image_pull.go

@@ -108,6 +108,7 @@ func (c *criService) PullImage(ctx context.Context, r *runtime.PullImageRequest)
 		containerd.WithPullSnapshotter(c.config.ContainerdConfig.Snapshotter),
 		containerd.WithPullUnpack,
 		containerd.WithPullLabel(imageLabelKey, imageLabelValue),
+		containerd.WithMaxConcurrentDownloads(c.config.MaxConcurrentDownloads),
 	)
 	if err != nil {
 		return nil, errors.Wrapf(err, "failed to pull and unpack image %q", ref)