File not found nginx php-fpm

The error indicates that your SCRIPT_FILENAME is incorrect. Your comment:

in the wordpress container it's at /var/www/html/index.php in the nginx container it's at /app

suggests that nginx and php-fpm are seeing a different document root.

In which case, use:

fastcgi_param   SCRIPT_FILENAME  /var/www/html$fastcgi_script_name;

You can try to include include snippets/fastcgi-php.conf;. It should fix your problem.