'Describes Amazon EC2 Container Service container instances.
Returns metadata about registered and remaining resources on
each container instance requested.
:type cluster: string
:param cluster: The short name or full Amazon Resource Name (ARN) of
the cluster that hosts the container instances you want to
describe. If you do not specify a cluster, the default cluster is
assumed.
:type container_instances: list
:param container_instances: A space-separated list of container
instance UUIDs or full Amazon Resource Name (ARN) entries.'

def describe_container_instances(self, container_instances, cluster=None):

    params = {}
    self.build_list_params(params, container_instances, 'containerInstances.member')
    if (cluster is not None):
        params['cluster'] = cluster
    return self._make_request(action='DescribeContainerInstances', verb='POST', path='/', params=params)
