cmd/tip: redirect from HTTP to HTTPS; update to Go 1.6

Change-Id: I7b219a991df4f71d068b62a22f69acb123ac31f0
Reviewed-on: https://go-review.googlesource.com/22367
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/tip/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.5
+FROM golang:1.6
 
 RUN apt-get update && apt-get install --no-install-recommends -y -q build-essential git
 

cmd/tip/tip.go

@@ -44,7 +44,7 @@ func main() {
 
 	p := &Proxy{builder: b}
 	go p.run()
-	http.Handle("/", p)
+	http.Handle("/", httpsOnlyHandler{p})
 	http.HandleFunc("/_ah/health", p.serveHealthCheck)
 
 	log.Print("Starting up")
@@ -323,3 +323,19 @@ func getOK(url string) (body []byte, err error) {
 	}
 	return body, nil
 }
+
+// httpsOnlyHandler redirects requests to "http://example.com/foo?bar"
+// to "https://example.com/foo?bar"
+type httpsOnlyHandler struct {
+	h http.Handler
+}
+
+func (h httpsOnlyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if r.Header.Get("X-Appengine-Https") == "off" {
+		r.URL.Scheme = "https"
+		r.URL.Host = r.Host
+		http.Redirect(w, r, r.URL.String(), http.StatusFound)
+		return
+	}
+	h.h.ServeHTTP(w, r)
+}