Use fpm image for Nextcloud

We are having some performance problems on ecloud.global, probably related to Apache which is included in the Nextcloud Docker image, and sometimes goes to 100% CPU usage for no reason. Using fastCGI instead should use less CPU and RAM while speeding up responses.

https://blog.layershift.com/which-php-mode-apache-vs-cgi-vs-fastcgi/

I already got this working on a test server, and subjectively, it felt much faster than with the current configuration. But right now it seems to be broken for some reason.

I also need to check if more nginx config items are needed (eg deny access to data folder).

https://github.com/nextcloud/docker/blob/master/.examples/docker-compose/with-nginx-proxy/mariadb/fpm/web/nginx.conf