I'm trying to start localstack in a containerized Jenkins build agent for unit testing. In other words, trying to start the localstack docker container from within a docker container. We do this all the time with other images, using the method of mounting `/var/run/docker.sock` in the "parent" container, but localstack will not start.

I get this error:

```
2019-11-08T18:53:53:ERROR:localstack.services.generic_proxy: Exception running proxy on port 8081: [Errno 13] Permission denied: '/tmp/localstack/server.test.pem' Traceback (most recent call last):
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 384, in run_cmd
    combined_file, cert_file_name, key_file_name = GenericProxy.create_ssl_cert(serial_number=self.port)
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 401, in create_ssl_cert
    return generate_ssl_cert(SERVER_CERT_PEM_FILE, serial_number=serial_number)
  File "/opt/code/localstack/localstack/utils/common.py", line 828, in generate_ssl_cert
    save_file(target_file, file_content)
  File "/opt/code/localstack/localstack/utils/common.py", line 642, in save_file
    with open(file, mode) as f:
PermissionError: [Errno 13] Permission denied: '/tmp/localstack/server.test.pem'
```

Port 8081 is not already in use, and `/tmp/localstack` is writable, so I don't understand why it's getting permission denied.
aws_stack.create_sqs_queue(self.TEST_SQS_QUEUE)
        
        result = connect_api_gateway_to_sqs('test_gateway4', stage_name=self.TEST_STAGE_NAME,
            queue_arn=self.TEST_SQS_QUEUE, path=self.API_PATH_DATA_INBOUND)
        
        test_data = {'spam': 'eggs'}
        url = INBOUND_GATEWAY_URL_PATTERN.format(
            api_id=result['id'],
            stage_name=self.TEST_STAGE_NAME,
            path=self.API_PATH_DATA_INBOUND
        )
        result = requests.post(url, data=json.dumps(test_data))
        self.assertEqual(result.status_code, 200)
        messages = aws_stack.sqs_receive_message(self.TEST_SQS_QUEUE)['Messages']
        self.assertEqual(len(messages), 1)
        self.assertEqual(json.loads(base64.b64decode(messages[0]['Body'])), test_data)
    def test_api_gateway_http_integration(self):
        test_port = 12123
        backend_url = 'http://localhost:%s%s' % (test_port, self.API_PATH_HTTP_BACKEND)
        
        class TestListener(ProxyListener):