📝 Adds Kubernetes Keycloak ingress rules into troubleshooting (gchq#479)

docs/troubleshooting.md

@@ -22,6 +22,7 @@
 - [Status Checks Failing](#status-checks-failing)
 - [Diagnosing Widget Errors](#widget-errors)
 - [Fixing Widget CORS Errors](#widget-cors-errors)
+- [Keycloak Redirect Error](#keycloak-redirect-error)
 - [How-To Open Browser Console](#how-to-open-browser-console)
 - [Git Contributions not Displaying](#git-contributions-not-displaying)
 
@@ -273,6 +274,25 @@ For testing purposes, you can use an addon, which will disable the CORS checks.
 
 ---
 
+## Keycloak Redirect Error
+
+Firstly, ensure that in your Keycloak instance you have populated the Valid Redirect URIs field ([screenshot](https://user-images.githubusercontent.com/1862727/148599768-db4ee4f8-72c5-402d-8f00-051d999e6267.png)) with the URL to your Dashy instance.
+
+You may need to specify CORS headers on your Keycloak instance, to allow requests coming from Dashy, e.g:
+
+```
+Access-Control-Allow-Origin: https://dashy.example.com
+```
+
+If you're running in Kubernetes, you will need to enable CORS ingress rules, see [docs](https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/annotations/#enable-cors), e.g:
+
+```
+nginx.ingress.kubernetes.io/cors-allow-origin: "https://dashy.example.com"
+nginx.ingress.kubernetes.io/enable-cors: "true"
+```
+
+---
+
 ## How-To Open Browser Console
 When raising a bug, one crucial piece of info needed is the browser's console output. This will help the developer diagnose and fix the issue.