More disk space for docker images in AppVM

How can I increase the disk space for my docker containers in an AppVM? I don’t want them persistent. So no bind-dirs are necessary.
I use a Debian 10 template.

I guess it is: /dev/xvda3
I tried both of these commands without success:

qvm-volume extend <name-of-app-vm>:private +20GB
qvm-volume extend <name-of-app-vm>:root +20GB

man qvm-volume
extend is an alias for resize, and you specify the NEW_SIZE

Also see in the docs: