Analyze and reduce clang:git container size

Container takes about 12 GB at the moment. We do include more than the normal clang installations of which some things are needed and nice but I assume lots of it is just trash that can be excluded either during the build (preferable) or at least excluded when creating the tar for transfer to the other layer.

Also we might think about other ways to copy the files to the target than using the tar archive which increases the image size even after removal (it still present in an intermediate layer).