Change Docker compose 'maintenance' port from '5000' to '3001'

- Because port '5000' has a standard use on macOS, it is said.

docker-compose.yml

@@ -105,7 +105,7 @@ services:
     networks:
       - external-net
     ports:
-      - 5000:80
+      - 3001:80
 
 networks:
   external-net:

webapp/maintenance/README.md

@@ -36,4 +36,4 @@ $ docker-compose up
 ````
 
 And the maintenance mode page or service will be started as well in an own container.
-In the browser you can reach it under `http://localhost:5000/`.
+In the browser you can reach it under `http://localhost:3001/`.