It looks like there is no way to specify a directory to build an image using the "dockerfile" option. It only supports "filename", which is analogous to "-f" for the "docker build" command-line, but "docker build" takes as a required argument the DIRECTORY. This is necessary if you don't want the docker daemon to copy the whole repository when making the image, and you want instead to export just a small subset of files. Maybe adding something like "build_args" could be more flexible.