Merge #1423

1423: Doc update: proxy_set_header required for admin r=mergify[bot] a=ofthesun9

Copy Paste of an existing PR whose upstream branch is gone
Fix #1318

## What type of PR?

documentation

## What does this PR do?

### Related issue(s)
closes #1318  (copy paste of 1318)

## Prerequistes
- [x ] In case of feature or enhancement: documentation updated accordingly
- [x ] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.


Co-authored-by: ofthesun9 <olivier@ofthesun.net>

docs/reverse.rst

@@ -61,6 +61,7 @@ Because the admin interface is served as ``/admin`` and the Webmail as ``/webmai
 
     location /admin {
       proxy_pass https://localhost:8443/admin;
+      proxy_set_header Host $http_host;
     }
 
     location /main_app {
@@ -103,6 +104,7 @@ Here is an example configuration :
 
     location /admin {
       proxy_pass https://localhost:8443/admin;
+      proxy_set_header Host $http_host;
     }
 
   }