Add Dockerfile for Selenium Firefox w/mp4 video support

docker/build/chrome/Dockerfile

+FROM selenium/standalone-chrome-debug:3.4.0-einsteinium
+MAINTAINER edxops
+
+USER root
+
+# Install a password generator
+RUN apt-get update -qqy \
+  && apt-get -qqy install \
+  pwgen \
+  && rm -rf /var/lib/apt/lists/* /var/cache/apt/*
+
+USER seluser
+
+CMD export VNC_PASSWORD=$(pwgen -s -1 $(shuf -i 10-20 -n 1)) \
+  && x11vnc -storepasswd $VNC_PASSWORD /home/seluser/.vnc/passwd \
+  && echo "Chrome VNC password: $VNC_PASSWORD" \
+  && /opt/bin/entry_point.sh
+
+EXPOSE 4444 5900

docker/build/firefox/Dockerfile

+FROM selenium/standalone-firefox-debug:3.4.0-einsteinium
+MAINTAINER edxops
+
+USER root
+
+# Install a password generator and the codecs needed to support mp4 video in Firefox
+RUN apt-get update -qqy \
+  && apt-get -qqy install \
+  gstreamer1.0-libav \
+  pwgen \
+  && rm -rf /var/lib/apt/lists/* /var/cache/apt/*
+
+USER seluser
+
+CMD export VNC_PASSWORD=$(pwgen -s -1 $(shuf -i 10-20 -n 1)) \
+  && x11vnc -storepasswd $VNC_PASSWORD /home/seluser/.vnc/passwd \
+  && echo "Firefox VNC password: $VNC_PASSWORD" \
+  && /opt/bin/entry_point.sh
+
+EXPOSE 4444 5900