FIX: CI docker image comes with graphviz now.

.docker/Dockerfile

@@ -3,7 +3,7 @@ FROM debian:buster-backports
 RUN apt-get update
 RUN apt-get install -y cmake/buster-backports
 RUN apt-get install -y lcov
-RUN apt-get install -y doxygen
+RUN apt-get install -y doxygen graphviz
 RUN apt-get install -y clang-format-11 clang-tidy-11
 RUN apt-get install -y python3-pip
 RUN apt-get install -y git