playground: specify memcache address via an env variable

This prevents those running on their own services from getting a
connection error if there is not a memcached server with that
exact name on the network.

Change-Id: Ib89630c33dc647979827d57fb40571cb21f462ab
Reviewed-on: https://go-review.googlesource.com/86249
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/app.yaml b/app.yaml
index 14b8cfd..77cba63 100644
--- a/app.yaml
+++ b/app.yaml
@@ -8,4 +8,7 @@
 
 health_check:
   check_interval_sec: 20
-  restart_threshold: 10
\ No newline at end of file
+  restart_threshold: 10
+
+env_variables:
+  MEMCACHED_ADDR: 'memcached:11211'
diff --git a/main.go b/main.go
index ee5bdb6..94a96bf 100644
--- a/main.go
+++ b/main.go
@@ -28,8 +28,11 @@
 			}
 			s.db = cloudDatastore{client: c}
 		}
-		if os.Getenv("GAE_INSTANCE") != "" {
-			s.cache = newGobCache("memcached:11211")
+		if caddr := os.Getenv("MEMCACHED_ADDR"); caddr != "" {
+			s.cache = newGobCache(caddr)
+			log.Printf("App (project ID: %q) is caching results", pid)
+		} else {
+			log.Printf("App (project ID: %q) is NOT caching results", pid)
 		}
 		s.log = log
 		return nil