Added instructions on how to combine varnish with basic-auth

Author: vidarl

docs/infrastructure_and_maintenance/cache/http_cache/reverse_proxy.md

@@ -142,6 +142,27 @@ ibexa:
                 purge_servers: [http://my.varnish.server:8081]
 ```
 
+#### Varnish and Basic Auth
+
+If purge requests to Varnish is protected by Basic Auth, the Basic Auth credentials can be specified with the
+`purge_servers` setting using the format:
+
+``` yaml
+            http_cache:
+                purge_servers: [http://myuser:[email protected]:8081]
+```
+
+Varnish is enabled by default when using [[= product_name_cloud =]]. There is no need to specify `purge_servers` as this is detected
+automatically. When enabling Basic Auth on [[= product_name_cloud =]] when using Varnish, you do however need to specify the credentials
+using the following environments in order for purging to work:
+
+```
+env:HTTPCACHE_USERNAME=myuser
+env:HTTPCACHE_PASSWORD=mypasswd
+```
+
+If you want to use Basic Auth with Fastly on [[= product_name_cloud =]], please see [Enable basic-auth on Fastly](...)
+
 !!! note "Invalidating Varnish cache using tokens"
 
     In setups where the Varnish server IP can change (for example, on [[= product_name_cloud =]]),