playground: differentiate cache key based on client JS version

Don't share a cache namespace between clients wanting vet (new) and
not wanting vet (old).

This was causing issues like:

   https://github.com/golang/go/issues/31944#issuecomment-493094932

And unrelated: a README deploy fix, broken from my earlier Makefile
cleanup.

Change-Id: Ibed7f91c739ac65e33ee8d73eed066a7a81f938c
Reviewed-on: https://go-review.googlesource.com/c/playground/+/177617
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

README.md

diff --git a/README.md b/README.md
index 6b8d71c..fe6ac19 100644
--- a/README.md
+++ b/README.md
@@ -56,7 +56,7 @@
 
 ```bash
 make docker
-docker tag playground:latest gcr.io/golang-org/playground:latest
+docker tag golang/playground:latest gcr.io/golang-org/playground:latest
 docker push gcr.io/golang-org/playground:latest
 gcloud --project=golang-org --account=you@google.com app deploy app.yaml --image-url=gcr.io/golang-org/playground:latest
 ```

sandbox.go

diff --git a/sandbox.go b/sandbox.go
index 2050832..f43eb07 100644
--- a/sandbox.go
+++ b/sandbox.go
@@ -74,6 +74,7 @@
 // The handler returned supports Cross-Origin Resource Sharing (CORS) from any domain.
 func (s *server) commandHandler(cachePrefix string, cmdFunc func(*request) (*response, error)) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
+		cachePrefix := cachePrefix // so we can modify it below
 		w.Header().Set("Access-Control-Allow-Origin", "*")
 		if r.Method == "OPTIONS" {
 			// This is likely a pre-flight CORS request.
@@ -92,6 +93,10 @@
 			return
 		}
 
+		if req.WithVet {
+			cachePrefix += "_vet" // "prog" -> "prog_vet"
+		}
+
 		resp := &response{}
 		key := cacheKey(cachePrefix, req.Body)
 		if err := s.cache.Get(key, resp); err != nil {