Made clangd available in docker image

Dockerfile

@@ -89,9 +89,12 @@ apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
 run \
 update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-9 100; \
 update-alternatives --install /usr/bin/clang clang /usr/bin/clang-9 100; \
+update-alternatives --install /usr/bin/clangd clangd /usr/bin/clangd-9 100; \
 update-alternatives --config clang; \
 update-alternatives --config clang++
 
+run update-alternatives --config clangd
+
 # this is done here so that ccache sees clang++ installed, not clang-9
 RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y ccache