<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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)