There is an environment variable that can be passed to LXD by adding an override in its systemd unit. LXD_IDMAPPED_MOUNTS_DISABLE=1
Alternatively, if it’s just for one path, say /var/lib/docker, you can create a storage volume using lxc storage volume create and then attach that to /var/lib/docker.
That volume will not be using idmap shifting unless you set security.shifted=true on it.