Expose local environments publicly

[shadyvb]

Developers often need a way to expose their local environments publicly, for testing purposes.

We can provide some documentation, and/or a command to expose a local environment publicly using ngrok, via a dedicated proxy container.

[roborourke]

Duplicate of humanmade/altis-local-server#335

[roborourke]

Note vagrant already has this feature, so we only need it for local server really.