This new Dockerfiles enables use to build images for Clang versions 10 and 11, which are not available in Ubuntu 18.04 via APT. Change-Id: I374a1c679377b7402a63241da34f9f4f7f5c7468 Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45681 Reviewed-by: Gabe Black <gabe.black@gmail.com> Maintainer: Gabe Black <gabe.black@gmail.com> Tested-by: kokoro <noreply+kokoro@google.com>