message: Add clang to the RBE GPU Dockerfile.
  Clang breaks with the libstdc++ version on ubuntu 14.04, so also install
  a newer libstdc++.
diff: # TODO(klimek): Include clang in this image so we can also target clang
  # builds.
  -FROM ubuntu:14.04
  +FROM gcr.io/clang-docker-builder/clang-ubuntu14_04
  LABEL maintainer="Manuel Klimek <klimek@google.com>"
  RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates apt-transport-https gnupg-curl && \
  @@ -71,6 +71,15 @@ RUN ln -s libcuda.so /usr/local/cuda/lib64/stubs/libcuda.so.1
  RUN ln -s /usr/lib/x86_64-linux-gnu/libnccl.so /usr/lib/libnccl.so \
  && ln -s /usr/lib/x86_64-linux-gnu/libnccl.so /usr/lib/libnccl.so.2
  +# Install a newer version of libstdc++, as new clang versions do not work
  +# with the stock ubuntu 14.04 libstdc++.
  +RUN apt-get update && \
  + apt-get install -y software-properties-common && \
  + add-apt-repository ppa:ubuntu-toolchain-r/test -y && \
  + apt-get update && \
  + apt-get install -y libstdc++-7-dev && \
  + rm -rf /var/lib/apt/lists/*
  +
  # Copy and run the install scripts.
  COPY install/*.sh /install/
  ARG DEBIAN_FRONTEND=noninteractive
  