I noticed the change because I wanted to persist Docker-related data between container instantiations on IncusOS. I couldn't understand why the custom volume I had mounted on /var/lib/docker didn't contain the downloaded images.

To keep both /var/lib/{containerd,docker} in sync, I use a single ZFS dataset ("custom filesystem volume" in Incus parlance) and mount subpaths inside the container:

  incus storage volume create local docker-data
  incus config device add docker docker disk pool=local source=docker-data/docker path=/var/lib/docker
  incus config device add docker containerd disk pool=local source=docker-data/containerd path=/var/lib/containerd

There are other ways to achieve the same of course.