[llvm] [Github][Docker] Remove 'docker.io/library' from image name (PR #162007)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> I thought it was considered best practice to use the full URI?

Is there a recommendation with motivation somewhere? I could understand a portability argument, but all of the container runtimes that we're using still seem to default to `docker.io`.

Either way is fine with me, but we should be consistent with what we have inside the repository.

https://github.com/llvm/llvm-project/pull/162007