Custom error page for v4

Where can I find the correct nginx conf file in order to add a command to use a custom 404 page?

I just want to add this code to it, but simply cant find the file which in a regular nginx instalation would be at : /etc/nginx

Command to add:

error_page 404 /var/lib/docker/volumes/<site>_htdocs/_data/htdocs/404.php;