Cache Go modules in platform builder

platform/api/router.go

@@ -53,6 +53,7 @@ func NewRouterFromConfig(cfg config.Config) (http.Handler, error) {
 		Image:        cfg.BuilderImage,
 		SourceDir:    cfg.BuilderSourceDir,
 		WorkspaceDir: cfg.BuilderWorkspaceDir,
+		CacheDir:     cfg.BuilderCacheDir,
 		Timeout:      time.Duration(cfg.BuilderTimeoutSeconds) * time.Second,
 	}))
 	if err := core.ConfigureAIProviderMode(cfg.AIProviderMode); err != nil {