Easily check logs outside of the container

Hola man :) thanks for this, it's sweet as hell!

I'm not a super  nginx/docker user so forgive me if I'm gonna open some dumb PRs. I am trying to make this work on our dev environments (which use SSL as well) and I have a bunch of things to send.

This is the first PR, lets you export the NGINX logs from wherever you launch the container.

My use case is that I usually launch the container from its repo itself and then I can simply do a `tail -f error.log` if I do anything wrong. There is probably a better way to do this but I didnt know how to play around with the `proc` stuff [here](https://github.com/jwilder/nginx-proxy/blob/master/nginx.tmpl#L4).

Cheers!