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

llvm-svn: 328503