Builds a docker image from the given docker context with a `Dockerfile` file object.

        :param ctx: An instance of :class:`~.context.DockerContext`.
        :type ctx: dockermap.build.context.DockerContext
        :param tag: New image tag.
        :type tag: unicode | str
        :param kwargs: See :meth:`docker.client.Client.build`.
        :return: New, generated image id or `None`.
        :rtype: unicode | str

def build_from_context(self, ctx, tag, **kwargs):
        """
        Builds a docker image from the given docker context with a `Dockerfile` file object.

        :param ctx: An instance of :class:`~.context.DockerContext`.
        :type ctx: dockermap.build.context.DockerContext
        :param tag: New image tag.
        :type tag: unicode | str
        :param kwargs: See :meth:`docker.client.Client.build`.
        :return: New, generated image id or `None`.
        :rtype: unicode | str
        """
        return self.build(fileobj=ctx.fileobj, tag=tag, custom_context=True, encoding=ctx.stream_encoding, **kwargs)