v0.3.0: Merge pull request #676 from tonistiigi/dockerfile-release

docker run --name buildkit -d --privileged -p 1234:1234 moby/buildkit:v0.3.0 --addr tcp://0.0.0.0:1234
# docker run --name buildkit -d --privileged -p 1234:1234 moby/buildkit:v0.3.0-rootless --addr tcp://0.0.0.0:1234
export BUILDKIT_HOST=tcp://0.0.0.0:1234
# docker cp buildkit:/usr/bin/buildctl /usr/local/bin/
buildctl build --help