Add NGINX Server to route requests

[dentonmwood]

Selenium Hub by default leaves port 4444 open, which would be fine if this wasn't Docker. We can network the nodes together with the hub, but we still want to leave the GUI open for user access. However, we don't want to publish the Selenium port to the world for others to access, which is exactly what Docker port publishing does. To fix this, we can reverse proxy with an NGINX server.

See this article for more info.