feat(setup/advanced): add kubernetes document

setup/advanced/kubernetes.md

@@ -0,0 +1,33 @@
+# Kubernetes
+
+## Common errors
+
+### `adding IPv6 rule: ...: file exists`
+
+This is caused by two factors:
+
+1. Kubernetes shares IP rules across the entire pod even if multiple Gluetun containers are running in the same pod
+1. Abrupt Gluetun exits which can be caused by a node shutdown, power event or just Gluetun hanging at shutdown (I'm working on fixing this)
+
+To resolve this, change the update strategy from `RollingUpdate` to `Recreate` strategy AND add post start command to ensure IP rules are deleted, for example for Wireguard:
+
+```yml
+apiVersion: apps/v1
+kind: Deployment
+spec:
+  strategy:
+    type: Recreate
+    spec:
+      containers:
+        - name: vpn
+          image: qmcgaw/gluetun
+          lifecycle:
+            postStart:
+              exec:
+                command: ["/bin/sh", "-c", "(ip rule del table 51820; ip -6 rule del table 51820) || true"]
+          # ...
+```
+
+**Credits to [@kvangent](https://github.com/kvangent)**
+
+See the [original issue #2521 comment which resolved this](https://github.com/qdm12/gluetun/issues/2521#issuecomment-2453592258)