Fixes to Dockerfile scripts.

- Put buildfiles into /tmp/clang-build/build, instead of /tmp/clang-build.
  We checkout the sources to /tmp/clang-build/src and running
  cmake in /tmp/clang-build was done by mistake.
- Don't add an extra ';' at the start of enabled projects list.
  It worked either way, but looked strange.
- Minor comment update.

llvm-svn: 307258