nginx-aspnet-mysql: add dev envs configuration (#267)

* nginx-aspnet-mysql: add dev envs configuration
* Fix/enable MySQL/MariaDB health check
* Refactor `Dockerfile` for use as Compose app as well as with
  dev envs
* nginx-aspnet-mysql: remove DB healthcheck from dev envs config

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