moby: Permissions problem on Docker 1.8 when reading from stdin

In my build scripts I have a line like so:

    tar c --mtime=2014-01-01 --exclude=__pycache__ --exclude=Dockerfile.template --mode="g=u,o=rX" --owner=root --group=root . | docker build --tag=$IMAGE_NAME:$BUILD_NUMBER -

It does work on Docker < 1.8 but on 1.8 it now throws:

unable to prepare context: unable to extract stdin to temporary context direcotry: lchown /tmp/docker-build-context-183505935/run_entrypoints.sh: operation not permitted

I can change it to use sudo:

        tar c --mtime=2014-01-01 --exclude=__pycache__ --exclude=*.pyc --exclude=Dockerfile.template --mode="g=u,o=rX" . | sudo docker build --tag=$IMAGE_NAME:$BUILD_NUMBER -

and then it works, but I wonder if it’s a bug or some change that affects permissions/users.