Fixes #123669

- Update Dockerfiles to work with the LLVM trunk
- Adapt Documentation accordingly
- Fix duplicate `-c` flag

to reflect the new license.

We understand that people may be surprised that we're moving the header
entirely to discuss the new license. We checked this carefully with the
Foundation's lawyer and we believe this is the correct approach.

Essentially, all code in the project is now made available by the LLVM
project under our new license, so you will see that the license headers
include that license only. Some of our contributors have contributed
code under our old license, and accordingly, we have retained a copy of
our old license notice in the top-level files in each project and
repository.

llvm-svn: 351636

Summary:
This is a small refactoring to extract the svn checkout code from the
build script used inside the docker image.
This would give more flexibility if more than a single invocation of
cmake is needed inside the docker image.

User-facing interface (build_docker_image.sh) hasn't changed, only the
internal scripts running inside the build container are affected.

Reviewers: ioeric

Reviewed By: ioeric

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D45868

llvm-svn: 330412

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