Hmm… A persistent run directory seems flaky to me. It will break apps that assume a tmpfs-mounted run directory. I think this is the main issue here - containerd does not cleanup the pid file; it assumes that the cleanup will happen automatically during reboot (not sure how it handles crashes).
Out of curiosity, docker-snap can already write to /var/run/docker.sock and /var/run/docker/, so why do we need a run directory under /var/snap/?