document the cekit version in use for building the OpenJDK images

If we try to rebuild an image source from a while ago and cekit has changed a lot in the meantime, the dist-git diff of the image sources might be huge (and impossible to review by a human) despite no image source changes. Therefore we should record which cekit version we are using for each release, and manage cekit version bumps (perhaps committing no-op image updates to dist-git to capture the delta caused purely by tooling changes)