[libc] Add docker container for llvm-libc buildbot worker.

Summary: Created a docker container to provide transparency and easy changes to the llvm-libc buildbot intfra.

Reviewers: sivachandra

Reviewed By: sivachandra

Subscribers: MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D75596