It looks like a duplicate of old issue https://github.com/localstack/localstack/issues/78
But now it's reproducible for me with the latest version of LocalStack. 
return SERVICE_PORTS.get(service_key, 0)
def external_service_url(service_key):
    return 'http%s://%s:%s' % ('s' if USE_SSL else '', HOSTNAME_EXTERNAL, service_port(service_key))
populate_configs()
if os.environ.get('DEBUG', '').lower() in TRUE_VALUES:
    logging.getLogger('').setLevel(logging.DEBUG)
    logging.getLogger('localstack').setLevel(logging.DEBUG)
BUNDLE_API_PROCESSES = True
USE_PROFILER = os.environ.get('USE_PROFILER', '').lower() in TRUE_VALUES
INBOUND_GATEWAY_URL_PATTERN = ('%s/restapis/{api_id}/{stage_name}/%s{path}' %
                               (TEST_APIGATEWAY_URL, PATH_USER_REQUEST))