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.
path_map = get_rest_api_paths(api_id)
        _, resource = get_resource_for_path('/lambda/foo1', path_map)
        
        path = path.replace('{test_param1}', 'foo1')
        path = path + '?foo=foo&bar=bar&bar=baz'
        url = INBOUND_GATEWAY_URL_PATTERN.format(
            api_id=api_id, stage_name=self.TEST_STAGE_NAME, path=path)
        data = {'return_status_code': 203, 'return_headers': {'foo': 'bar123'}}
        result = requests.post(url, data=json.dumps(data),
            headers={'User-Agent': 'python-requests/testing'})
        self.assertEqual(result.status_code, 203)
        self.assertEqual(result.headers.get('foo'), 'bar123')
        parsed_body = json.loads(to_str(result.content))
        self.assertEqual(parsed_body.get('return_status_code'), 203)
        self.assertDictEqual(parsed_body.get('return_headers'), {'foo': 'bar123'})
        self.assertDictEqual(parsed_body.get('queryStringParameters'), {'foo': 'foo', 'bar': ['bar', 'baz']})
        request_context = parsed_body.get('requestContext')
        source_ip = request_context['identity'].pop('sourceIp')
        self.assertTrue(re.match(r'^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$', source_ip))
        self.assertEqual(request_context['path'], '/lambda/foo1')
        self.assertEqual(request_context['accountId'], TEST_AWS_ACCOUNT_ID)
        self.assertEqual(request_context['resourceId'], resource.get('id'))
        self.assertEqual(request_context['stage'], self.TEST_STAGE_NAME)
        self.assertEqual(request_context['identity']['userAgent'], 'python-requests/testing')
        result = requests.delete(url, data=json.dumps(data))
        self.assertEqual(result.status_code, 404)
        
        body_msg = '🙀 - 参よ'
        result = requests.post(url, data=json.dumps({'return_raw_body': body_msg}))
        self.assertEqual(to_str(result.content), body_msg)