rego-tunnel: detect container IP by subnet instead of interface name

Interface order is not consistent. Search for 172.31.x.x subnet instead.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/rego-tunnel/vpn_scripts/start.sh

@@ -28,8 +28,8 @@ get_windows_ip() {
 }
 
 get_container_ip() {
-  # Get container's external IP (vpn_static-rego - eth0)
-  ip -4 addr show eth0 2>/dev/null | awk '/inet / {print $2}' | cut -d/ -f1
+  # Get container's external IP on 172.31.x.x subnet (vpn_static-rego)
+  ip -4 addr 2>/dev/null | grep -oE '172\.31\.[0-9]+\.[0-9]+' | head -1
 }
 
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