Attempt to fix Xvfb-run crash when running selenium images

Dockerfile

# replace with the specific selenium image you use
# the problem is with Xvfb, not the selenium server or the browser,
# so this fix should apply to all selenium images
FROM selenium/standalone-firefox

# attempt to fix this issue:
# https://github.com/SeleniumHQ/docker-selenium/issues/91

# switch to root so you can chmod the file
USER root

# this assumes you've set $DISPLAY to :99.0 (this is the default value, so if
# not sure please do not modify the line below)
RUN rm -rf /tmp/.X99-lock && chmod +x /opt/bin/entry_point.sh

# switch back to seluser after the chmod
USER seluser