Add external volume

compose.mailman.yml

@@ -2,10 +2,17 @@ version: "3.6"
 
 services:
   smtp:
+    volumes:
+      - "mailman-core:/opt/mailman/"
     configs:
       - source: postfix_override
         target: /overrides/postfix.cf
 
+volumes:
+  mailman-core:
+    # https://git.autonomic.zone/coop-cloud/mailman3/src/branch/master/compose.yml
+    external: true
+
 configs:
   postfix_override:
     name: ${STACK_NAME}_postfix_overrides_${POSTFIX_OVERRIDE_VERSION}