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.
GATEWAY_RESPONSES[api_id] = GATEWAY_RESPONSES.get(api_id, [])
    data['responseType'] = response_type
    GATEWAY_RESPONSES[api_id].append(data)
    return data
def invoke_rest_api(api_id, stage, method, invocation_path, data, headers, path=None):
    path = path or invocation_path
    relative_path, query_string_params = extract_query_string_params(path=invocation_path)
    path_map = helpers.get_rest_api_paths(rest_api_id=api_id)
    try:
        extracted_path, resource = get_resource_for_path(path=relative_path, path_map=path_map)
    except Exception:
        return make_error('Unable to find path %s' % path, 404)
    integrations = resource.get('resourceMethods', {})
    integration = integrations.get(method, {})
    if not integration:
        integration = integrations.get('ANY', {})
    integration = integration.get('methodIntegration')
    if not integration:
        if method == 'OPTIONS' and 'Origin' in headers:
            
            return get_cors_response(headers)
        return make_error('Unable to find integration for path %s' % path, 404)
    uri = integration.get('uri')