Adds clangd to the docker container

Containers/dockerfiles/Ubuntu20_04.Dockerfile

@@ -11,6 +11,7 @@
 # freeGLUT
 # cppcheck
 # clang 10.0 (default)
+# clangd language server https://clangd.llvm.org/
 # python pip3 with modules: setuptools, wheels, scikit-build, pyvista, numpy, ansible, gcovr
 
 FROM nvidia/cuda:11.3.1-devel-ubuntu20.04
@@ -33,6 +34,8 @@ RUN apt-get update &&   \
     python3-pip         \
     freeglut3-dev       \
     cppcheck            \
+    clangd-12           \
+    && update-alternatives --install /usr/bin/clangd clangd /usr/bin/clangd-12 100 \
     && mkdir -p /usr/local/cmake/ && cd /usr/local/cmake/ \
     && version=3.22 && build=3 \
     && wget https://cmake.org/files/v$version/cmake-$version.$build-linux-x86_64.tar.gz \