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.
data_str = json.dumps(data) if isinstance(data, (dict, list)) else data
            account_id = uri.split(':lambda:path')[1].split(':function:')[0].split(':')[-1]
            source_ip = headers['X-Forwarded-For'].split(',')[-2]
            
            
            request_context = {
                'path': relative_path,
                'accountId': account_id,
                'resourceId': resource.get('id'),
                'stage': stage,
                'identity': {
                    'accountId': account_id,
                    'sourceIp': source_ip,
                    'userAgent': headers['User-Agent'],
                }
            }
            try:
                path_params = extract_path_params(path=relative_path, extracted_path=extracted_path)
            except Exception:
                path_params = {}
            result = lambda_api.process_apigateway_invocation(func_arn, relative_path, data_str,
                headers, path_params=path_params, query_string_params=query_string_params,
                method=method, resource_path=path, request_context=request_context)
            if isinstance(result, FlaskResponse):
                return flask_to_requests_response(result)
            if isinstance(result, Response):
                return result
            response = Response()
            parsed_result = result if isinstance(result, dict) else json.loads(result)
            parsed_result = common.json_safe(parsed_result)