Container images and related state is stored in /var/db/containers. It
is recommended to use ZFS for this:

# zfs create -o mountpoint=/var/db/containers zroot/containers

If your system cannot use ZFS, change storage.conf to use the vfs
storage driver:

# sed -I .bak -e 's/driver = "zfs"/driver = "vfs"/' /usr/local/etc/containers/storage.conf