go2go: add an app engine service to redirect go2goplay to gotipplay

This service will overwrite the existing go2go playground.

Updates golang/go#48517

Change-Id: Idc9e6733cc213a2c3e84c71d1c4746dc31feeb62
Reviewed-on: https://go-review.googlesource.com/c/playground/+/364858
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

Author: findleyr

app.go2go.yaml

@@ -0,0 +1,10 @@
+runtime: go116
+service: go2goplay
+main: ./cmd/redirect
+
+handlers:
+- url: /.*
+  script: auto
+
+env_variables:
+  PLAY_REDIRECT: "https://gotipplay.golang.org"

cmd/redirect/main.go

@@ -0,0 +1,32 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// redirect serves an http server that redirects to the URL specified by the
+// environment variable PLAY_REDIRECT.
+package main
+
+import (
+	"log"
+	"net/http"
+	"os"
+)
+
+func main() {
+	port := os.Getenv("PORT")
+	if port == "" {
+		port = "8080"
+	}
+
+	redirect := os.Getenv("PLAY_REDIRECT")
+	if redirect == "" {
+		redirect = "https://play.golang.org"
+	}
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, redirect+r.URL.Path, http.StatusFound)
+	})
+
+	log.Printf("Listening on :%v ...", port)
+	log.Fatalf("Error listening on :%v: %v", port, http.ListenAndServe(":"+port, handler))
+}