Added GUI instructions for unattended cluster mode

Author: ArneTR

content/en/docs/measuring/measuring-gui-applications.md

@@ -7,7 +7,9 @@ weight: 839
 toc: true
 ---
 
-GMT can also measure GUI applications. We only support Linux atm but will update this page once further OS are supported.
+GMT can also measure GUI applications that need an active Window Manager (X11 / Wayland etc).
+
+We only support Linux atm but will update this page once further OS are supported.
 
 ### Linux
 
@@ -91,5 +93,30 @@ Theoretically macOS should be possible if you install *X Server* on your system.
 
 Let us know if you try it out and it works!
 
+### Automating for cluster
+
+If you are running GMT unattended in cluster mode you must ensure two things:
+- The user that the GMT runs with must be automatically also logged into a GUI session. This typically happens in the user settings of the desktop (for instance Gnome).
+- Futhermore the `xhost +local:docker` must be set also in the Desktop. In an unattended mode this must happen with files that are loaded via autostart.
+
+#### X11 autostart file
+Add `xhost +local:docker` to `~/.xprofile`
+
+Example: `$ xhost +" >> ~/.xprofile`
+
+#### Wayland autostart file
+Create a file `~/.config/autostart/xhost-docker.desktop`
+
+Content:
+```config
+[Desktop Entry]
+Type=Application
+Exec=xhost +local:docker
+Hidden=false
+NoDisplay=false
+X-GNOME-Autostart-enabled=true
+Name=Allow Docker X11 / Wayland
+```
+
 ### Help / Debugging
 If you run into any errors see the [Debugging →]({{< relref "debugging" >}}) page.