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.
SIGNAL_HANDLERS_SETUP = False
DEFAULT_BACKEND_HOST = '127.0.0.1'
LOG = logging.getLogger(os.path.basename(__file__))
SERVICE_PLUGINS = {}
class Plugin(object):
    def __init__(self, name, start, check=None, listener=None):
        self.plugin_name = name
        self.start_function = start
        self.listener = listener
        self.check_function = check
    def start(self, asynchronous):
        kwargs = {
            'asynchronous': asynchronous
        }
        if self.listener:
            kwargs['update_listener'] = self.listener
        return self.start_function(**kwargs)
    def check(self, expect_shutdown=False, print_error=False):
        if not self.check_function:
            return
        return self.check_function(expect_shutdown=expect_shutdown, print_error=print_error)
    def name(self):
        return self.plugin_name
def register_plugin(plugin):
    SERVICE_PLUGINS[plugin.name()] = plugin
def update_config_variable(variable, new_value):
    if new_value is not None:
        LOG.info('Updating value of config variable "%s": %s' % (variable, new_value))
        setattr(config, variable, new_value)
class ConfigUpdateProxyListener(ProxyListener):