Open stdin when using Veloren with docker.

This allows entering CLI commands like `help`, `admin add`, and
`shutdown` directly using `docker attach`, rather than needing to run
server-cli commands separately or perform a manual reboot or interrupt.

server-cli/docker-compose.yml

@@ -3,6 +3,7 @@ version: "3.5"
 services:
   game-server:
     image: registry.gitlab.com/veloren/veloren/server-cli:nightly
+    stdin_open: true
     container_name: veloren-game-server-master
     ports:
       - "14004:14004"