Add docker image for ffmpeg

.gitlab-ci.yml

 pages:
-  image: boileaum/jupyter
+  image: boileaum/jupyter:ffmpeg
   script:
     - pip install --user -r requirements.txt
     - nbcourse -n 5

docker/Dockerfile

+FROM boileaum/jupyter
+
+USER root
+RUN apt-get update && apt-get -yq dist-upgrade \
+ && apt-get install -yq --no-install-recommends ffmpeg \
+ && apt-get clean \
+ && rm -rf /var/lib/apt/lists/*
+
+USER jovyan
\ No newline at end of file