[Dockerfiles] Split checkout and build scripts into separate files.

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