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.
""" Default proxy listener that intercepts requests to retrieve or update config variables. """
    def forward_request(self, method, path, data, headers):
        if path != constants.CONFIG_UPDATE_PATH or method != 'POST':
            return True
        response = Response()
        data = json.loads(data)
        variable = data.get('variable', '')
        response._content = '{}'
        response.status_code = 200
        if not re.match(r'^[_a-zA-Z0-9]+$', variable):
            response.status_code = 400
            return response
        new_value = data.get('value')
        update_config_variable(variable, new_value)
        value = getattr(config, variable, None)
        result = {'variable': variable, 'value': value}
        response._content = json.dumps(result)
        return response
GenericProxyHandler.DEFAULT_LISTENERS.append(ConfigUpdateProxyListener())
def start_apigateway(port=None, asynchronous=False, update_listener=None):