Ensure Xvfb is only started once the watcher is running

Dockerfile

@@ -127,8 +127,10 @@ RUN \
     export DISPLAY=$display; \
     mkdir -p --mode=777 /tmp/.X11-unix; \
     # Create job for waiting for Xvfb to start
-    { inotifywait -e create /tmp/.X11-unix/ 2>&1 > /dev/null & }; \
+    watchStarting="$(mktemp)"; \
+    { 2>&1 inotifywait -e create /tmp/.X11-unix/; } | { sed -u -e "/^Watches established.\$/e rm \"$watchStarting\""; } & \
     resolver=$!; \
+    while [ -f "$watchStarting" ]; do true; done; \
     { \
         # Improve arguments for screenshots
         ARGS=$([ $DEBUG -eq 1 ] && echo "-screen 0 640x480x24 -dpi 192" || echo ""); \