chore(docs): optionally install XVFB in docker

Author: aslushnikov

docs/docker/Dockerfile.bionic

@@ -46,5 +46,8 @@ RUN groupadd -r pwuser && useradd -r -g pwuser -G audio,video pwuser \
     && mkdir -p /home/pwuser/Downloads \
     && chown -R pwuser:pwuser /home/pwuser
 
+# 6. (Optional) Install XVFB if there's a need to run browsers in headful mode
+RUN apt-get install -y xvfb
+
 # Run everything after as non-privileged user.
 USER pwuser