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.
parsed_result = {} if parsed_result is None else parsed_result
            response.status_code = int(parsed_result.get('statusCode', 200))
            response.headers.update(parsed_result.get('headers', {}))
            try:
                if isinstance(parsed_result['body'], dict):
                    response._content = json.dumps(parsed_result['body'])
                else:
                    response._content = to_bytes(parsed_result['body'])
            except Exception:
                response._content = '{}'
            response.headers['Content-Length'] = len(response._content)
            return response
        else:
            msg = 'API Gateway action uri "%s" not yet implemented' % uri
            LOGGER.warning(msg)
            return make_error(msg, 404)
    elif integration['type'] == 'HTTP':
        function = getattr(requests, method.lower())
        if isinstance(data, dict):
            data = json.dumps(data)
        result = function(integration['uri'], data=data, headers=headers)
        return result
    else:
        msg = ('API Gateway integration type "%s" for method "%s" not yet implemented' %
               (integration['type'], method))
        LOGGER.warning(msg)
        return make_error(msg, 404)
    return 200
UPDATE_APIGATEWAY = ProxyListenerApiGateway()