centos: mount point should be one directory up

[gnimmagadda]

For centos builds
docker run --privileged -v $(pwd):/workspace builder /build.sh

mounting $(pwd) results in base-images-docker/centos but build.sh looks for: tar -C /target -cf /workspace/centos/layer.tar .

and it fails because /workspace/centos dir doesn't exist (host side will be centos/centos).

Either need to mount one dir up $(pwd)/..:/workspace or cd to one dir and to start the builder.