Use xdummy if possible

config/skel/.xsessionrc

@@ -1,4 +1,5 @@
-export VGL_DISPLAY=${DISPLAY:?}
+#!/bin/sh
+
 export XDG_CACHE_HOME=${HOME:?}/.cache
 export XDG_CONFIG_DIRS=/etc/xdg/xdg-xubuntu:/etc/xdg
 export XDG_CONFIG_HOME=${HOME:?}/.config
@@ -6,6 +7,10 @@ export XDG_CURRENT_DESKTOP=XFCE
 export XDG_DATA_DIRS=/usr/share/xubuntu:/usr/share/xfce4:/usr/local/share:/usr/share
 export XDG_DATA_HOME=${HOME:?}/.local/share
 export XDG_MENU_PREFIX=xfce-
-export XDG_RUNTIME_DIR=/run/user/$(id -u)
+export XDG_RUNTIME_DIR=/run/user/${UNPRIVILEGED_USER_UID:?}
 export XDG_SESSION_DESKTOP=xubuntu
 export XDG_SESSION_TYPE=x11
+
+if [ "${ENABLE_XDUMMY:?}" != 'true' ]; then
+	export VGL_DISPLAY=${DISPLAY:?}
+fi