[TASK] Adjust symfony nginx config to doc

[afoeder]

This adjusts the nginx configuration to the one that is mentioned in the Symfony documentation.

[sandrokeil]

@afoeder I mean something like this for the Symfony stuff. So we need no new config file for Symfony, just replace root and main PHP file.

FROM prooph/nginx:www

RUN sed -i "s#index.php#app.php#g" /etc/nginx/sites-enabled/www.conf

[codeliner]

that's no longer needed for symfony 4.x, right?