id is a string describing the runtime, e.g 'flashgames

        Returns a configured DockerRuntime object

def runtime_spec(self, id):
        """
        id is a string describing the runtime, e.g 'flashgames

        Returns a configured DockerRuntime object
        """
        try:
            return self.runtimes[id]
        except KeyError:
            raise UnregisteredRuntime('No registered runtime with name: {}'.format(id))