Summary:
- Removed double indirection via command-line args (i.e. two `--`
options of `build_docker_image.sh`).
- Added a comment on how to build 2-stage clang install into the
`build_docker_image.sh`, it used to be only in the `docs/Docker.rst`.
Reviewers: klimek, mehdi_amini
Reviewed By: klimek
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D35050
llvm-svn: 307256