Allow GC to discard content after successful pull and unpack

This commit adds a config flag for allowing GC to clean layer contents up after
unpacking these contents completed, which leads to deduplication of layer
contents between the snapshotter and the contnet store.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

pkg/server/image_pull.go

@@ -127,6 +127,12 @@ func (c *criService) PullImage(ctx context.Context, r *runtime.PullImageRequest)
 			containerd.WithImageHandlerWrapper(appendInfoHandlerWrapper(ref)))
 	}
 
+	if c.config.ContainerdConfig.DiscardUnpackedLayers {
+		// Allows GC to clean layers up from the content store after unpacking
+		pullOpts = append(pullOpts,
+			containerd.WithChildLabelMap(containerdimages.ChildGCLabelsFilterLayers))
+	}
+
 	image, err := c.client.Pull(ctx, ref, pullOpts...)
 	if err != nil {
 		return nil, errors.Wrapf(err, "failed to pull and unpack image %q", ref)