message: add ninja-build package to Dockerfile
diff: @@ -22,7 +22,7 @@ RUN set -e; \
  RUN set -e; \
  apt-get -y --no-install-recommends install libtool \
  # compilers (gcc-7, gcc-9, clang-6)
  - build-essential clang-6.0 lldb-6.0 lld-6.0 g++-9 \
  + build-essential clang-6.0 lldb-6.0 lld-6.0 g++-9 ninja-build \
  # CI dependencies
  git ssh tar gzip ca-certificates gnupg \
  # Pythons
  