Add support for running a container using only a XvFB framebuffer for the X server

As requested by a user, this gives the option to run a headless steam for the purpose of downloading games and not for playing them.

overlay/etc/supervisor.d/xvfb.ini

@@ -0,0 +1,13 @@
+
+[program:xvfb]
+autostart=false
+autorestart=true
+priority=20
+user=root
+command=/usr/bin/Xvfb %(ENV_DISPLAY)s -screen 0 "%(ENV_DISPLAY_SIZEW)sx%(ENV_DISPLAY_SIZEH)sx24"
+environment=DISPLAY="%(ENV_DISPLAY)s"
+stopsignal=QUIT
+stdout_logfile_maxbytes=10MB
+stdout_logfile_backups=7
+stderr_logfile_maxbytes=10MB
+stderr_logfile_backups=7