fix(builder): use a local node-agent image when it can't be pulled

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/gbrgames/firstboot.go

@@ -146,8 +146,8 @@ func runFirstBoot(args []string) error {
 			return err
 		}
 	}
-	if out, err := runCmd("docker", "pull", "-q", b.agentImage); err != nil {
-		return fmt.Errorf("docker pull %s: %v: %s", b.agentImage, err, out)
+	if err := pullAgentImage(b.agentImage); err != nil {
+		return err
 	}
 	if !*keep {
 		defer b.cleanup()