Fixing bootstrapping with gcc.

Turns out it didn't work anymore to bootstrap with gcc because
that can't compile libc++abi. *sigh*.

Added a phase 0 that bootstraps a clang without using
libc++/libc++abi. That will then be used in phase 1 and 2. Phase zero
is skipped if (any) clang is available already.
Adding Dockerfile.

This builds an Ubuntu image with LLVM/clang installed into /opt/llvm.
Updating README.
Add Docker ignore file.