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>

cmd/gbrgames/import.go

@@ -18,7 +18,7 @@ import (
 // (enabled, allowPaidHosting, allowPrivateHosting, imageUrl) are deliberately left out.
 var gameFields = []string{
 	"id", "name",
-	"defaultDockerImage", "defaultStartupCommand", "requiredMemoryMb", "requiredDiskMb", "defaultPorts",
+	"defaultDockerImage", "containerUser", "defaultStartupCommand", "requiredMemoryMb", "requiredDiskMb", "defaultPorts",
 	"installMethod", "installConfig",
 	"defaultConfig", "configSchema", "configFilePath", "configFileFormat", "configFormatOptions",
 	"agnosticCommandConfig", "clusterConfig",