code: def build_from_context(self, ctx, tag, **kwargs):
          
          return self.build(fileobj=ctx.fileobj, tag=tag, custom_context=True, encoding=ctx.stream_encoding, **kwargs)
docs: 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