Uses internetsystemsconsortium/bind9 as the default base image instead of
a self-hosted one, verified against internetsystemsconsortium/bind9:9.20
(runs as root; named/rndc/nsupdate at /usr/sbin,/usr/sbin,/usr/bin).
- project operator config at /etc/bind-operator instead of overmounting
the image's /etc/bind (keeps bind.keys / base config intact)
- reference named/rndc/nsupdate by absolute path (exec PATH may exclude
/usr/sbin)
- centralise filesystem + binary paths in internal/bind/consts.go
- default spec.image to internetsystemsconsortium/bind9:9.20
