message: Add ability to override branch
diff: +# This script can be run with the following command:
  +# docker build --build-arg branch=<your branch here, defaults to develop> -t "axom/tpls:clang-10" - < dockerfile_clang-10
  +
  FROM axom/compilers:clang-10
  +ARG branch=develop
  # Update base image so we don't have to install so many things via Spack
  SHELL ["/bin/bash", "-c"]
  @@ -11,7 +15,7 @@ WORKDIR "/home/axom"
  USER axom
  # Clone axom
  -RUN git clone -b feature/white238/addclang10docker --recursive https://github.com/LLNL/axom.git
  +RUN git clone -b $branch --recursive https://github.com/LLNL/axom.git
  # Build TPLs
  WORKDIR "/home/axom/axom"
  