Add some missing XDG environment variables

Dockerfile.debian

@@ -190,7 +190,8 @@ RUN \
 # TODO: Refine this list of packages to only what is required.
 ENV \
     XORG_SOCKET_DIR="/tmp/.X11-unix" \
-    XDG_RUNTIME_DIR="/tmp/.X11-unix/run"
+    XDG_RUNTIME_DIR="/tmp/.X11-unix/run" \
+    XDG_SESSION_TYPE="x11"
 RUN \
     echo "**** Update apt database ****" \
         && apt-get update \

overlay/usr/bin/start-desktop.sh

@@ -25,7 +25,10 @@ rm -f /tmp/.started-desktop
 # Note: This script should be the only one that waits for X after exporting this dbus session
 rm -fv /tmp/.dbus-desktop-session.env
 export_desktop_dbus_session
-
+# Configure some XDG environment variables
+export XDG_CACHE_HOME="${USER_HOME:?}/.cache"
+export XDG_CONFIG_HOME="${USER_HOME:?}/.config"
+export XDG_DATA_HOME="${USER_HOME:?}/.local/share"
 
 # EXECUTE PROCESS:
 # Wait for the X server to start