[libc++] Add a utility script to run the Docker image used by builders

Several contributors have been asking me how to reproduce the CI
environment locally. This is the last step towards making that work
out-of-the-box. Basically, just run `libcxx/utils/ci/run-buildbot-container`
and you're good to go.

Differential Revision: https://reviews.llvm.org/D97782

GitOrigin-RevId: c5659dd4cc84ec1e46b038c19c62ebb6051bd3c4
2 files changed