Currently the information provided by is not describing the implementation, but I can add it if you want. Do you prefer the old or new version?

http://docs.docker.io/en/latest/api/docker_remote_api_v1.4/#build-an-image-from-dockerfile-via-stdin
http://docs.docker.io/en/latest/api/docker_remote_api_v1.8/#build-an-image-from-dockerfile-via-stdin