Update Dockerfile with only clang/llvm release 9

Dockerfile

@@ -7,4 +7,4 @@ ADD import-llvm-keys.sh /root/import-llvm-keys.sh
 RUN sh /root/import-llvm-keys.sh
 RUN echo "deb http://apt.llvm.org/stretch/ llvm-toolchain-stretch main" > /etc/apt/sources.list.d/llvm.list
 RUN apt update
-RUN apt install -y -qq clang lldb lld clang-tools-8
+RUN apt install -y -qq clang-9 clang-tools-9 clang-9-doc libclang-common-9-dev libclang-9-dev libclang1-9 clang-format-9 python-clang-9 clangd-9 libllvm9 llvm-9 llvm-9-dev llvm-9-doc llvm-9-examples llvm-9-runtime libfuzzer-9-dev