Made a script to build docker images easier to use.

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