Include optional custom nginx config files (#72)

If you want to control e.g. the ssl_protocols or ssl_ciphers settings,
create a file named 'custom-ssl.inc' in the /home/labca/nginx_data/conf.d/
directory (or when using the docker-only setup, either add a volume mount
or edit the labca_nginx_conf volume to include the file). Similarly,
'custom-base.inc' can be created to e.g. define a custom log format, and
'custom.inc' could be created for any settings on the plain HTTP server
context.

proxy.inc

@@ -0,0 +1,4 @@
+proxy_set_header Host $http_host;
+proxy_set_header X-Real-IP $remote_addr;
+proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+proxy_set_header X-Forwarded-Proto $scheme;