message: ci: add lcov to Docker image.
  This provides genhtml, which is needed for Bazel native coverage report
  generation.
diff: @@ -6,7 +6,7 @@ set -e
  apt-get update
  export DEBIAN_FRONTEND=noninteractive
  apt-get install -y wget software-properties-common make cmake git python python-pip python3 python3-pip \
  - unzip bc libtool ninja-build automake zip time golang gdb strace wireshark tshark tcpdump
  + unzip bc libtool ninja-build automake zip time golang gdb strace wireshark tshark tcpdump lcov
  # clang 7.
  wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -
  apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-7 main"
  