client, ctr: allow specifying unmanaged rootfs dir

e.g. ctr run -t --rm --rootfs /tmp/busybox-rootfs foo /bin/sh
(--rm removes the container but does not remove rootfs dir, of course)

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

cmd/ctr/run.go

@@ -47,7 +47,7 @@ func withMounts(context *cli.Context) containerd.SpecOpts {
 var runCommand = cli.Command{
 	Name:      "run",
 	Usage:     "run a container",
-	ArgsUsage: "IMAGE CONTAINER [COMMAND] [ARG...]",
+	ArgsUsage: "Image|RootFS ID [COMMAND] [ARG...]",
 	Flags: append([]cli.Flag{
 		cli.BoolFlag{
 			Name:  "tty,t",