Docker storage driver: UCR variable not working

Hi, I am currently migrating some Docker instances from aufs to overlay2 to make them ready for upcoming Debian Bullseye upgrades (which does not support aufs any more). Since UCS 5 is based on Debian (I know, it’s still Buster), I also had a look at the situation there.

Most of my UCS 4 systems already use overlay2, except one which somehow still uses overlay. I found the UCR variable docker/daemon/default/opts/storage-driver which is actually set to overlay on all systems, even if they are using overlay2. Changing the variable to overlay2 on that one system still using overlay and restarting Docker doesn’t change anything. The variable changes DOCKER_OPTS in /etc/default/docker, but I think the recommended way to change the storage driver is via /etc/docker/daemon.json (then it is working at least).

To make it persistent, I added a line "storage-driver": configRegistry["docker/daemon/default/opts/storage-driver"], to /etc/univention/templates/files/etc/docker/daemon.json, but this file could be overwritten by an update. It would be nice, if UCS fixes the template and the UCR variable (should there really be a default here, why not just let Docker decide?).