serving/helloworld-go: Respect the PORT env var (#457)

Author: grayside

serving/samples/helloworld-go/README.md

@@ -42,11 +42,16 @@ following instructions recreate the source files from this folder.
     }
 
     func main() {
-      flag.Parse()
       log.Print("Hello world sample started.")
 
       http.HandleFunc("/", handler)
-      http.ListenAndServe(":8080", nil)
+
+      port := os.Getenv("PORT")
+      if port == "" {
+        port = "8080"
+      }
+
+      log.Fatal(http.ListenAndServe(fmt.Sprintf(":%s", port), nil))
     }
     ```
 

serving/samples/helloworld-go/helloworld.go

@@ -36,5 +36,11 @@ func main() {
 	log.Print("Hello world sample started.")
 
 	http.HandleFunc("/", handler)
-	http.ListenAndServe(":8080", nil)
+
+	port := os.Getenv("PORT")
+	if port == "" {
+		port = "8080"
+	}
+
+	log.Fatal(http.ListenAndServe(fmt.Sprintf(":%s", port), nil))
 }