cmd/containerboot: only wipeout serve config when TS_SERVE_CONFIG is set

Fixes #9558

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/containerboot/main.go

@@ -258,11 +258,14 @@ authLoop:
 	if err := tailscaleSet(ctx, cfg); err != nil {
 		log.Fatalf("failed to auth tailscale: %v", err)
 	}
-	// Remove any serve config that may have been set by a previous
+
-	// run of containerboot.
+	if cfg.ServeConfigPath != "" {
+		// Remove any serve config that may have been set by a previous run of
+		// containerboot, but only if we're providing a new one.
 		if err := client.SetServeConfig(ctx, new(ipn.ServeConfig)); err != nil {
 			log.Fatalf("failed to unset serve config: %v", err)
 		}
+	}
 
 	if cfg.InKubernetes && cfg.KubeSecret != "" && cfg.KubernetesCanPatch && cfg.AuthOnce {
 		// We were told to only auth once, so any secret-bound