nginx-flask-mysql: add dev envs support (#272)

* 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

Co-authored-by: Guillaume Lours <guillaume@lours.me>
Signed-off-by: Milas Bowman <milas.bowman@docker.com>