Add a volume to contain the minecraft world files

etc/nixos/services/minecraft.nix

@@ -121,6 +121,7 @@ in
         "25565:25565"
       ];
       volumes = [
+        "minecraft:/var/lib/minecraft/world"
         "${ops}:/var/lib/minecraft/ops.json:ro"
         "${whitelist}:/var/lib/minecraft/whitelist.json:ro"
       ];