Improvements to Debian Docker image build.

Add changes from Arch dockerfile:
- Flatpak support
- Reduce unnecessary recommends installations
- Separate mesa and vulkin installations

overlay/etc/home_directory_template/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml

@@ -27,22 +27,22 @@
         <property name="workspace0" type="empty">
           <property name="color-style" type="int" value="1"/>
           <property name="image-style" type="int" value="5"/>
-          <property name="last-image" type="string" value="/usr/share/images/desktop-base/default"/>
+          <property name="last-image" type="string" value="/usr/share/backgrounds/steam.jpg"/>
         </property>
         <property name="workspace1" type="empty">
           <property name="color-style" type="int" value="1"/>
           <property name="image-style" type="int" value="5"/>
-          <property name="last-image" type="string" value="/usr/share/images/desktop-base/default"/>
+          <property name="last-image" type="string" value="/usr/share/backgrounds/steam.jpg"/>
         </property>
         <property name="workspace2" type="empty">
           <property name="color-style" type="int" value="1"/>
           <property name="image-style" type="int" value="5"/>
-          <property name="last-image" type="string" value="/usr/share/images/desktop-base/default"/>
+          <property name="last-image" type="string" value="/usr/share/backgrounds/steam.jpg"/>
         </property>
         <property name="workspace3" type="empty">
           <property name="color-style" type="int" value="1"/>
           <property name="image-style" type="int" value="5"/>
-          <property name="last-image" type="string" value="/usr/share/images/desktop-base/default"/>
+          <property name="last-image" type="string" value="/usr/share/backgrounds/steam.jpg"/>
         </property>
       </property>
     </property>