Problem: containerd is running even docker is disabled

Problem

You deactivated the docker service by using the ucr variable but after a reboot the containerd service is still running. Service dockerd is stopped, though.
ucr set docker/autostart=no

Solution

Related to docker there are two services: dockerd and containerd. The first requires the second to run.
The ucr variable just disables dockerd but in order to save ressources containerd can be disabled, too.

systemctl disable containerd