command line flags of setting container rootfs propagation

Signed-off-by: Kitt Hsu <kitt.hsu@gmail.com>

cmd/ctr/commands/commands_unix.go

@@ -36,5 +36,8 @@ func init() {
 	}, cli.Uint64Flag{
 		Name:  "cpu-period",
 		Usage: "Limit CPU CFS period",
+	}, cli.StringFlag{
+		Name:  "rootfs-propagation",
+		Usage: "set the propagation of the container rootfs",
 	})
 }