Force start mailman inside the container.

Even though the 'master.lck' is supposed to prevent race conditions, it needs to
be manually removed if the container did not shutdown gracefully. Remove the
manual check for the lock file and force mailman to remove it.

Also, set the stop_grace_period to 30s, which would allow Core to stop
gracefully and not leave the lock file over. Default value in case of Docker is 10s.

docker-compose.yaml

@@ -7,6 +7,7 @@ services:
     hostname: mailman-core
     volumes:
     - /opt/mailman/core:/opt/mailman/
+    stop_grace_period: 30s
     links:
     - database:database
     depends_on: