Use overlay for writeable containers

Previously we would sneakily remount as `rw` but of course you can't
really do that on a truly immutable filesystem.

See https://github.com/moby/tool/pull/129 for the `moby` side.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/runc/cmd/onboot/main.go

@@ -56,6 +56,9 @@ func main() {
 		if err := cmd.Run(); err != nil {
 			log.Printf("Error running %s: %v", name, err)
 			status = 1
+		} else {
+			// do not clean up on error to make debug easier
+			cleanup(fullPath)
 		}
 	}