Deb: fix Ubuntu Focal (vnc_startup.sh XFCE start issue)

builder/dockerfile.ubuntu_focal.deb.test

@@ -45,6 +45,9 @@ ARG KASMVNC_PACKAGE_DIR
 COPY $KASMVNC_PACKAGE_DIR/kasmvncserver_*.deb /tmp
 RUN dpkg -i /tmp/*.deb; apt-get -yf install
 
+RUN mkdir ~/.vnc && echo '/usr/bin/xfce4-session &' >> ~/.vnc/xstartup && \
+  chmod +x ~/.vnc/xstartup
+
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