Problems with free storage calculation in docker

Hi, is there a way in nextcloud to define the path were the free space is calculated? I run nextcloud with docker and have an extra volume for /var/www/html with xfs backing fs and -o size=?G. The problem i am facing now that probably the free root storage is calculated and not the free storage from the volume. The default root storage is 20GB in docker. But if my volume is 5GB on the admin page the bar shows maybe 10%, but it should be much more. Is there a solution?