Just to finalize, it was NOT necessary to modify the GRUB_CMDLINE_LINUX value in /etc/default/grub but it IS necessary to manually run the script from here after each reboot: https://github.com/moby/moby/issues/8791#issuecomment-60874893.
It appears that even though cgroupfs-mount is required by the docker.io package, either the cgroupfs-mount package or else the docker.io package is not setting things up properly with the cgroup system mounts and they are not getting mounted at boot time.
