fix(ark): run as the image's own user; honour custom RCON ports

ARK's image is built around user gameserver (uid 25000) and locks
/home/gameserver to it, so under the node's default 1000:1000 the
startup script got "Permission denied" and the server restart-looped,
on real nodes as well as in boot tests. Set containerUser 25000:25000
(needs node-agent and panel support for containerUser).

The startup command read ${RCON_PORT}, but the node passes the RCONPort
setting as RCONPORT, so custom RCON ports were ignored for 27020.

Boot-tested in the Linux agent image: up 30 minutes with no restart.
The test now waits 10 minutes, covering the first ~13 GB download.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

schema/game.schema.json

@@ -16,6 +16,11 @@
     "name": { "description": "Display name shown to users.", "type": "string", "minLength": 1 },
 
     "defaultDockerImage": { "description": "Image the game server runs in.", "type": "string", "minLength": 1 },
+    "containerUser": {
+      "description": "uid:gid the server runs as, for images built around their own user (ARK's needs 25000:25000). Defaults to 1000:1000; root is never allowed.",
+      "type": ["string", "null"],
+      "pattern": "^[1-9][0-9]{0,8}:[1-9][0-9]{0,8}$"
+    },
     "defaultStartupCommand": { "description": "Command run inside the container. May use config placeholders.", "type": ["string", "null"] },
     "requiredMemoryMb": { "type": ["integer", "null"], "minimum": 128 },
     "requiredDiskMb": { "type": ["integer", "null"], "minimum": 128 },