docs(env): add custom redis variables (#412)

Author: Meierschlumpf

docs/advanced/environment-variables/index.mdx

@@ -8,6 +8,7 @@ tags:
   - Environment variables
   - Variables
   - Database
+  - Redis
 ---
 
 Homarr offers a Docker Container, which can be run on any compatible system, such as Unraid, Kubernetes and many more systems! Our Docker container is based on the ``22.12.0-alpine`` image and serves per standard on the port `7575`.
@@ -70,6 +71,25 @@ A random secret can be generated by using the following command: `openssl rand -
 You can either use the url or host, port, name and credentials combined. The URL will be prioritized over the other values.
 
 
+## Redis
+
+By default Redis is running within the installation. (for example in Docker).
+However for example for K8s it can be useful to use an external Redis instance.
+
+:::note
+External redis is currently not supported with Proxmox Community Scripts
+:::
+
+| Environment Variable     | Description | Possible values | Default |
+| ------------------------ | ----------- | --------------- | ------- |
+| `REDIS_IS_EXTERNAL`     | Whether Redis is running externally. | `true` / `false` | `false` |
+| `REDIS_HOST`            | Hostname of the Redis instance. | Any valid hostname | - |
+| `REDIS_PORT`            | Port of the Redis instance. | Any valid port | `6379` |
+| `REDIS_USERNAME`        | Username to connect to the Redis instance. | Any valid username | - |
+| `REDIS_PASSWORD`        | Password to connect to the Redis instance. | Any valid password | - |
+| `REDIS_DATABASE_INDEX`  | Select which database of your redis instance should be used. | Any numeric index | - |
+| `REDIS_TLS_CA`         | CA certificate for Redis TLS connections. If a certificate is specified the connection will use TLS. | Any valid CA certificate | - |
+
 
 ## Advanced deployments