Docker: Add rsync and graphviz to system requirements

The former is used for the backup functionality and the latter is needed
to generate graphic representations of provenance graphs.

Cherry-pick: bf631f1

.docker/aiida-core-base/Dockerfile

@@ -40,6 +40,8 @@ RUN apt-get update --yes && \
     # development tools
     git \
     openssh-client \
+    rsync \
+    graphviz \
     vim \
     # the gcc compiler need to build some python packages e.g. psutil and pymatgen
     build-essential \