devops(docker): Install ffmpeg dependency, adding codecs necessary fo…

…r video playback in Firefox (#1627)

Fixes #1613

docs/docker/Dockerfile.bionic

@@ -41,12 +41,16 @@ RUN apt-get install -y libnss3 \
 
 RUN apt-get install -y libdbus-glib-1-2
 
-# 5. Add user so we don't need --no-sandbox in Chromium
+# 5. Install ffmpeg to bring in audio and video codecs necessary for playing videos in Firefox.
+
+RUN apt-get install -y ffmpeg
+
+# 6. Add user so we don't need --no-sandbox in Chromium
 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
+# 7. (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.