Add shared network config

compose.mailman.yml

@@ -2,12 +2,20 @@ version: "3.6"
 
 services:
   smtp:
+    networks:
+      - default
+      - shared_mailman_network
     volumes:
       - "shared-mailman-core:/opt/mailman/"
     configs:
       - source: postfix_override
         target: /overrides/postfix.cf
 
+networks:
+  shared_mailman_network:
+    external: true
+    name: ${MAILMAN_CORE_NETWORK}
+
 volumes:
   # https://git.autonomic.zone/coop-cloud/mailman3/src/branch/master/compose.yml
   shared-mailman-core: