Merge branch 'feature/pin-clang-format-version' into 'master'

Pin the `clang-format` version to `9`

See merge request docker-images/gr-build-images!6

code-style-check/Dockerfile

 FROM debian:sid
 LABEL maintainer="Ingo Heimbach <i.heimbach@fz-juelich.de>"
 
-ARG CLANG_FORMAT_VERSION="8"
+ARG CLANG_FORMAT_VERSION="9"
 ARG CMAKE_FORMAT_VERSION="0.6.7"
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends "clang-format-${CLANG_FORMAT_VERSION}" file git python3-pip \
                                                python3-setuptools python3-wheel && \
+    update-alternatives --install "/usr/bin/clang-format" \
+                                  "clang-format" \
+                                  "/usr/bin/clang-format-${CLANG_FORMAT_VERSION}" \
+                                  1 && \
     python3 -m pip install "cmake_format==${CMAKE_FORMAT_VERSION}" && \
-    rm -rf /var/lib/apt/lists/* && \
-    ln -s /usr/bin/clang-format-${CLANG_FORMAT_VERSION} /usr/local/bin/clang-format
+    rm -rf /var/lib/apt/lists/*
 
 CMD ["/bin/bash"]