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 forward_request(self, method, path, data, headers):
        data = data and json.loads(to_str(data))
        if re.match(PATH_REGEX_USER_REQUEST, path):
            search_match = re.search(PATH_REGEX_USER_REQUEST, path)
            api_id = search_match.group(1)
            stage = search_match.group(2)
            relative_path_w_query_params = '/%s' % search_match.group(3)
            return invoke_rest_api(api_id, stage, method, relative_path_w_query_params, data, headers, path=path)
        if re.match(PATH_REGEX_AUTHORIZERS, path):
            return handle_authorizers(method, path, data, headers)
        if re.match(PATH_REGEX_RESPONSES, path):
            search_match = re.search(PATH_REGEX_RESPONSES, path)
            api_id = search_match.group(1)
            if method == 'GET':
                return get_gateway_responses(api_id)
            if method == 'PUT':
                response_type = search_match.group(2).lstrip('/')
                return put_gateway_response(api_id, response_type, data)
        return True
    def return_response(self, method, path, data, headers, response):