support using multiple snapshotters simultaneously

e.g. dist pull --snapshotter btrfs ...; ctr run --snapshotter btrfs ...
(empty string defaults for overlayfs)

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/ctr/run_unix.go

@@ -106,6 +106,7 @@ func newContainer(ctx gocontext.Context, client *containerd.Client, context *cli
 			containerd.WithSpec(spec),
 			containerd.WithImage(image),
 			containerd.WithContainerLabels(labels),
+			containerd.WithSnapshotter(context.String("snapshotter")),
 			rootfs,
 		)
 	}