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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> 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?

We added a `pull_request` event to it and then conditioned the actual upload on being a `push` event. So it builds the container and runs through everything in a pull request modifying it, but only actually uploads and deploys when it gets pushed into main.

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