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/build.go

@@ -255,8 +255,8 @@ func firstBoot(name, image, steamApp string, steam bool, startup string) (*bootR
 // bootCheck deploys the template with the node-agent's BootTest, the same check CI runs, and when the
 // game wrote files outside its volume on the first boot, boots it once more to see that it no longer does.
 func bootCheck(gamesDir, id, agentImage string, wantOutside bool) (problems, verified []string, err error) {
-	if out, err := runCmd("docker", "pull", "-q", agentImage); err != nil {
-		return nil, nil, fmt.Errorf("docker pull %s: %v: %s", agentImage, err, out)
+	if err := pullAgentImage(agentImage); err != nil {
+		return nil, nil, err
 	}
 	output, passed, problems, err := agentBoot(gamesDir, id, agentImage, false)
 	if err != nil || problems != nil {