[DOCS] Add FAQs to Traefik2 (#1038)

Closes #997.

docs/deployment/supported-proxies/traefik2.x.md

@@ -107,4 +107,21 @@ services:
       - TZ=Australia/Melbourne
 ```
 
+## FAQ
+
+### Middleware authelia@docker not found
+
+If Traefik and Authelia are defined in different docker compose stacks you may experience
+an issue where Traefik complains that: `middleware authelia@docker not found`.
+
+This can be avoided a couple different ways:
+1. Ensure Authelia container is up before Traefik is started:
+    - Utilise the [`depends_on` option](https://docs.docker.com/compose/compose-file/#depends_on)
+2. Define the Authelia middleware on your Traefik container
+```yaml
+- 'traefik.http.middlewares.authelia.forwardauth.address=http://authelia:9091/api/verify?rd=https://login.example.com/'
+- 'traefik.http.middlewares.authelia.forwardauth.trustForwardHeader=true'
+- 'traefik.http.middlewares.authelia.forwardauth.authResponseHeaders=Remote-User, Remote-Groups'
+```
+    
 [Traefik 2.x]: https://docs.traefik.io/