Make EX_APP_CONTAINER_PREFIX configurable

[small1]

Describe why it is important and where it will be useful

If you have several Nextcloud instances on the same host. It would be good to be able to run several proxys that has its own prefix configured on the same host.

Right now it is nc_apps and it make it impossible to register two proxies on the same host (for development puproses, testing and simpler hosted solutions)

Describe your proposed solution

Either have this as a variable within the web interface or as an occ setting. Create a prefix for the current instance.

Describe alternatives you've considered, if relevant

No response

Additional context

No response