replace hard coded solr dns with dynamic variable

docker-compose.yml

@@ -399,7 +399,7 @@ services:
       volumes:
         - solr-vol-1:/opt/solr/server/solr/dovecot/data
       dns:
-        - 172.22.1.254
+        - ${IPV4_NETWORK:-172.22.1}.254
       dns_search: mailcow-network
       environment:
         - SOLR_HEAP=${SOLR_HEAP:-1024}