Symlink wont work, as /var/lib/egroupware is bind-mounted into EGroupware and Nginx containers.
So you have two possibilites:
a) Change the bin-mount / volume in the docker-compose file to the mount-point of your disk
b) Change the mount-point of your disk to /var/lib/egroupware
I strongly recommend b) as modifying the docker-compose file makes it harder for you on the long run, when we update it again eg. to add further containers.