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.
if integration['type'] == 'AWS':
        if 'kinesis:action/' in uri:
            if uri.endswith('kinesis:action/PutRecords'):
                target = kinesis_listener.ACTION_PUT_RECORDS
            if uri.endswith('kinesis:action/ListStreams'):
                target = kinesis_listener.ACTION_LIST_STREAMS
            template = integration['requestTemplates'][APPLICATION_JSON]
            new_request = aws_stack.render_velocity_template(template, data)
            
            headers = aws_stack.mock_aws_request_headers(service='kinesis')
            headers['X-Amz-Target'] = target
            result = common.make_http_request(url=TEST_KINESIS_URL,
                method='POST', data=new_request, headers=headers)
            return result
        if method == 'POST':
            if uri.startswith('arn:aws:apigateway:') and ':sqs:path' in uri:
                template = integration['requestTemplates'][APPLICATION_JSON]
                account_id, queue = uri.split('/')[-2:]
                region_name = uri.split(':')[3]
                new_request = aws_stack.render_velocity_template(template, data) + '&QueueName=%s' % queue
                headers = aws_stack.mock_aws_request_headers(service='sqs', region_name=region_name)
                url = urljoin(TEST_SQS_URL, '%s/%s' % (account_id, queue))
                result = common.make_http_request(url, method='POST', headers=headers, data=new_request)
                return result
        msg = 'API Gateway AWS integration action URI "%s", method "%s" not yet implemented' % (uri, method)
        LOGGER.warning(msg)
        return make_error(msg, 404)
    elif integration['type'] == 'AWS_PROXY':
        if uri.startswith('arn:aws:apigateway:') and ':lambda:path' in uri:
            func_arn = uri.split(':lambda:path')[1].split('functions/')[1].split('/invocations')[0]