Or you can avoid that complexity if you use the “cloud” variant, as cloud-init sorts this for you.
# incus launch images:alpine/edge/cloud foo --vm -c security.secureboot=false -d root,size=15GiB
# incus exec foo -- df -k /
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda2 15068900 316228 14736288 2% /