c8d/builder: Set empty diffID for rolayer

diffID is the digest of a tar archive containing changes to the parent
layer - rolayer doesn't have any changes to the parent.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

daemon/containerd/image_builder.go

@@ -201,7 +201,7 @@ func newROLayerForImage(ctx context.Context, imgDesc *ocispec.Descriptor, i *Ima
 		key:                key,
 		c:                  i.client,
 		snapshotter:        i.snapshotter,
-		diffID:             digest.Digest(parent),
+		diffID:             "", // Image RO layer doesn't have a diff.
 		contentStoreDigest: "",
 	}, nil
 }