[llvm] [libc++] Add a Github action to build libc++'s Docker images (PR #110020)

[Louis Dionne via llvm-commits]

ldionne wrote:

@tstellar Do you know how I could test this? How did you test `.github/workflows/build-ci-container.yml` when you first came up with it?

https://github.com/llvm/llvm-project/pull/110020