For running Docker, your best bet is to create an additional volume on your ZFS using block mode and attach that.
Basically:
incus storage volume create default docker size=50GiB zfs.block_mode=true block.filesystem=ext4
incus config device add YOUR-INSTANCE docker disk source=docker pool=default path=/var/lib/docker
ZFS 2.2 is nice because it supports VFS idmap so you get faster container creation and no need for dealing with the whole shifting business (even though that’s mostly hidden away by Incus anyways). But older ZFS will still do fine for this particular case.