Update maxking/mailman-core Docker tag to v0.5

This MR contains the following updates:

Package	Update	Change
maxking/mailman-core	minor	0.4 -> 0.5

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testserver/docker_mail/templates/docker-compose.yml

@@ -166,7 +166,7 @@ services:
 
   # Additional Services: mailman
   mailman-core:
-    image: maxking/mailman-core:0.4
+    image: maxking/mailman-core:0.5
     restart: always
     env_file: mailman.env
     depends_on: