docker-pi-hole/src/s6/debian-root/etc/lighttpd/conf-enabled/15-pihole-admin-redirect-docker.conf

$HTTP["url"] == "/" {
    $HTTP["host"] == env.VIRTUAL_HOST {
         url.redirect = ("" => "/admin/")
    }
    $HTTP["host"] == env.FTLCONF_LOCAL_IPV4 {
         url.redirect = ("" => "/admin/")
    }
}