[llvm] [libc++] Add Docker images to support incoming pre-merge CI transition (PR #144597)

[Louis Dionne via llvm-commits]

ldionne wrote:

@cmtice Once this is checked in, individual Docker images can be obtained with:

```
curl -s https://raw.githubusercontent.com/llvm/llvm-project/main/.github/workflows/libcxx-docker-images.yml | yq '.libcxx-runner-set-current'
```


https://github.com/llvm/llvm-project/pull/144597