[BUG]: Increase pm.max_children setting for Nextcloud

[pmig]

Actual:

For Nextclouds with many current users pm.max_children exhausts. The default value (5) should be overwritten, as following log statement is printed:

[pool www] server reached pm.max_children setting (5)

Desired:

Add or overwrite a php.ini file in /usr/local/etc/php-fpm.d/ with a value better suited to the target environment. We can use 256 as starting value.

Other:

An open Nextcloud pr would make this configureavle via an environment variable: nextcloud/docker#1766