fn: pre-fork pool for namespace/network speedup (#874)

* fn: pre-fork pool experimental implementation

api/agent/agent.go

@@ -129,7 +129,10 @@ func createAgent(da DataAccess, withDocker bool) Agent {
 	var driver drivers.Driver
 	if withDocker {
 		driver = docker.NewDocker(drivers.Config{
-			ServerVersion: cfg.MinDockerVersion,
+			ServerVersion:   cfg.MinDockerVersion,
+			PreForkPoolSize: cfg.PreForkPoolSize,
+			PreForkImage:    cfg.PreForkImage,
+			PreForkCmd:      cfg.PreForkCmd,
 		})
 	} else {
 		driver = mock.New()
@@ -154,12 +157,16 @@ func (a *agent) Enqueue(ctx context.Context, call *models.Call) error {
 }
 
 func (a *agent) Close() error {
+	var err error
 	a.shutonce.Do(func() {
+		if a.driver != nil {
+			err = a.driver.Close()
+		}
 		close(a.shutdown)
 	})
 
 	a.wg.Wait()
-	return nil
+	return err
 }
 
 func (a *agent) Submit(callI Call) error {