Migrate dockerfiles to use multi-stage builds.
Summary:
We previously emulated multi-staged builds using two dockerfiles,
native support from Docker allows us to merge them into one,
simplifying our scripts.
For more details about multi-stage builds, see:
https://docs.docker.com/develop/develop-images/multistage-build/
Reviewers: mehdi_amini, klimek, sammccall
Reviewed By: sammccall
Subscribers: llvm-commits, ioeric, cfe-commits
Differential Revision: https://reviews.llvm.org/D44787
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328503
91177308-0d34-0410-b5e6-
96231b3b80d8