Disallow creation of local external storage

[szaimen]

nextcloud/server#30111
Also add documentation to the readme that this is the case then

Todo:
Discuss with lukas beforehand if we should add an option to mount the /media and/or /mnt directory into the nextcloud container (Would probably then not necessarily run on otherplatforms like macos) #76

[szaimen]

Could even think about adding this with the next 23 release if it gets merged in time: nextcloud/server#31481