Resolve "Fix default parameters for the periodic bubble example"

Closes #25 (closed)

ci/docker_images/ubuntu/bionic/Dockerfile

@@ -209,6 +209,8 @@ RUN cd /tmp                                      \
 # ensure all libraries are known by the runtime linker
 RUN ldconfig
 
+RUN apt-get update && apt-get install -y python-tk
+
 # clean cached packages
 RUN rm -rf /var/lib/apt/lists/*
 RUN rm -rf $HOME/.cache/pip/*