Add clang-format to build image.

Dockerfile-ubuntu1804

     wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add - ; \
     apt update -y && \
     apt-get install -y clang-10 lldb-10 lld-10 && \
-    apt-get install -y libc++-10-dev libc++-10-dev && \
+    apt-get install -y libc++-10-dev libc++-10-dev clang-format-10 && \
     \
     rm -rf /var/lib/apt/lists/*