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 base64
import re
import json
import unittest
from requests.models import Response
from xml.dom.minidom import parseString
from localstack.config import INBOUND_GATEWAY_URL_PATTERN, DEFAULT_REGION
from localstack.constants import TEST_AWS_ACCOUNT_ID
from localstack.utils import testutil
from localstack.utils.aws import aws_stack
from localstack.utils.common import to_str, load_file
from localstack.utils.common import safe_requests as requests
from localstack.services.generic_proxy import GenericProxy, ProxyListener
from localstack.services.awslambda.lambda_api import (
    LAMBDA_RUNTIME_PYTHON27, add_event_source)
from localstack.services.apigateway.helpers import (
    get_rest_api_paths, get_resource_for_path, connect_api_gateway_to_sqs)
from .test_lambda import TEST_LAMBDA_PYTHON, TEST_LAMBDA_LIBS
class TestAPIGatewayIntegrations(unittest.TestCase):
    
    APIGATEWAY_DATA_INBOUND_TEMPLATE = """{
        "StreamName": "%s",
        "Records": [
            #set( $numRecords = $input.path('$.records').size() )
            #if($numRecords > 0)