Simple nginx reverse proxy.

A simple nginx reverse proxy, which adds headers that allow us to track
request queuing. It expects to be linked to an `app` container which
proxied requests should be forwarded to. It accepts environment
variables:

 - `$NGINX_PORT` - The port nginx should listen on.
 - `$APP_PORT` - The port of `app` that requests should be forwarded to,
   i.e. the port the application is bound to.

It expects to be run as a side-car.

It combines the configuration of:
 - https://github.com/awslabs/ecs-nginx-reverse-proxy/blob/master/reverse-proxy/
 - https://github.com/deliveroo/rs-devices/