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.
def test_api_gateway_lambda_proxy_integration_any_method(self):
        self._test_api_gateway_lambda_proxy_integration_any_method(
            self.TEST_LAMBDA_PROXY_BACKEND_ANY_METHOD,
            self.API_PATH_LAMBDA_PROXY_BACKEND_ANY_METHOD)
    def test_api_gateway_lambda_proxy_integration_any_method_with_path_param(self):
        self._test_api_gateway_lambda_proxy_integration_any_method(
            self.TEST_LAMBDA_PROXY_BACKEND_ANY_METHOD_WITH_PATH_PARAM,
            self.API_PATH_LAMBDA_PROXY_BACKEND_ANY_METHOD_WITH_PATH_PARAM)
    def _test_api_gateway_lambda_proxy_integration_any_method(self, fn_name, path):
        self.create_lambda_function(fn_name)
        
        lambda_uri = aws_stack.lambda_function_arn(fn_name)
        target_uri = aws_stack.apigateway_invocations_arn(lambda_uri)
        result = self.connect_api_gateway_to_http_with_lambda_proxy(
            'test_gateway3', target_uri, methods=['ANY'], path=path)
        
        path = path.replace('{test_param1}', 'foo1')
        url = INBOUND_GATEWAY_URL_PATTERN.format(
            api_id=result['id'], stage_name=self.TEST_STAGE_NAME, path=path)
        data = {}
        for method in ('GET', 'POST', 'PUT', 'PATCH', 'DELETE', 'OPTIONS'):
            body = json.dumps(data) if method in ('POST', 'PUT', 'PATCH') else None
            result = getattr(requests, method.lower())(url, data=body)
            self.assertEqual(result.status_code, 200)
            parsed_body = json.loads(to_str(result.content))
            self.assertEqual(parsed_body.get('httpMethod'), method)
    # =====================================================================
    # Helper methods
    # =====================================================================
    def connect_api_gateway_to_kinesis(self, gateway_name, kinesis_stream):