rootless: bail if euid != 0

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

cmd/buildkitd/main.go

@@ -135,6 +135,9 @@ func main() {
 	app.Flags = append(app.Flags, appFlags...)
 
 	app.Action = func(c *cli.Context) error {
+		if os.Geteuid() != 0 {
+			return errors.New("rootless mode requires to be executed as the mapped root in a user namespace; you may use RootlessKit for setting up the namespace")
+		}
 		ctx, cancel := context.WithCancel(appcontext.Context())
 		defer cancel()