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.
import re
import json
import logging
import requests
from flask import Response as FlaskResponse
from six.moves.urllib_parse import urljoin
from requests.models import Response
from localstack.constants import APPLICATION_JSON, PATH_USER_REQUEST
from localstack.config import TEST_KINESIS_URL, TEST_SQS_URL
from localstack.utils import common
from localstack.utils.aws import aws_stack
from localstack.utils.common import to_str, to_bytes
from localstack.utils.analytics import event_publisher
from localstack.services.kinesis import kinesis_listener
from localstack.services.awslambda import lambda_api
from localstack.services.apigateway import helpers
from localstack.services.generic_proxy import ProxyListener
from localstack.utils.aws.aws_responses import flask_to_requests_response, requests_response
from localstack.services.apigateway.helpers import (get_resource_for_path,
    handle_authorizers, extract_query_string_params,
    extract_path_params, make_error, get_cors_response)
LOGGER = logging.getLogger(__name__)
PATH_REGEX_AUTHORIZERS = r'^/restapis/([A-Za-z0-9_\-]+)/authorizers(\?.*)?'
PATH_REGEX_RESPONSES = r'^/restapis/([A-Za-z0-9_\-]+)/gatewayresponses(/[A-Za-z0-9_\-]+)?(\?.*)?'
PATH_REGEX_USER_REQUEST = r'^/restapis/([A-Za-z0-9_\-]+)/([A-Za-z0-9_\-]+)/%s/(.*)$' % PATH_USER_REQUEST
GATEWAY_RESPONSES = {}
class ProxyListenerApiGateway(ProxyListener):