nginx-flask-mysql: add dev envs support

* Add Docker Desktop Development Environments config
* Change port `5000` -> `8000` for Flask to avoid conflicts on
  recent macOS versions
* Improve DB health check (for non-dev envs case) to avoid
  producing a bunch of log spam

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

nginx-flask-mysql/proxy/conf

@@ -2,7 +2,7 @@ server {
     listen       80;
     server_name  localhost;
     location / {
-        proxy_pass   http://backend:5000;
+        proxy_pass   http://backend:8000;
     }
 
 }