Add `NGINX_ACCESS_LOG` and `NGINX_ERROR_LOG`

[robsontenorio]

This is my first contribution. Let me know if something can be adjusted.

Targeting release/v3.6

I kept the default current values (/dev/stdout and /dev/stderr). But, honestly I would change it to the Nginx defaults. Because, it is not useful to have all that logs on the main stdout. But if you don't think so, it is all good to be merged.

Usage

FROM serversideup ...
ENV NGINX_ACCESS_LOG=/var/log/nginx/access.log
ENV NGINX_ERROR_LOG=/var/log/nginx/error.log