HAProxy: update for Pi-Hole

images/rpi-haproxy/haproxy.cfg

     use_backend letsencrypt-backend if letsencrypt-acl
     use_backend nextcloud-insecure if is_nextcloud
     use_backend gogs-insecure if is_gogs
-    default_backend nextcloud-insecure
+    default_backend pihole-insecure
 
 backend nextcloud-insecure
     server nextcloud nextcloud:80 maxconn 32
 backend gogs-insecure
     server gogs gogs:2080 maxconn 32
 
+backend pihole-insecure
+    server pihole pihole:80 maxconn 32
+
 # LE Backend
 backend letsencrypt-backend
     server letsencrypt 127.0.0.1:8888