This patch fixes a bug that is still in the ubuntu image at dockerhub.
xvfb-run merged stdout and stderr.

https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1059947
This bug was fixed in the package xorg-server - 2:1.19.4-1ubuntu2

--- xvfb-run   2019-02-28 06:18:21.000000000 +0000
+++ xvfb-run   2019-03-04 12:37:31.129377336 +0000
@@ -178,7 +178,7 @@
 
 # Start the command and save its exit status.
 set +e
-DISPLAY=:$SERVERNUM XAUTHORITY=$AUTHFILE "$@" 2>&1
+DISPLAY=:$SERVERNUM XAUTHORITY=$AUTHFILE "$@"
 RETVAL=$?
 set -e