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.
custom_backend_url = os.environ.get('%s_BACKEND' % service_name.upper())
    backend_url = custom_backend_url or ('http://%s:%s' % (DEFAULT_BACKEND_HOST, default_backend_port))
    return start_proxy(port, backend_url=backend_url, update_listener=update_listener, quiet=quiet, params=params)
def start_proxy(port, backend_url, update_listener=None, quiet=False, params={}):
    proxy_thread = GenericProxy(port=port, forward_url=backend_url,
        ssl=config.USE_SSL, update_listener=update_listener, quiet=quiet, params=params)
    proxy_thread.start()
    TMP_THREADS.append(proxy_thread)
    return proxy_thread
def start_moto_server(key, port, name=None, backend_port=None, asynchronous=False, update_listener=None):
    if not name:
        name = key
    print('Starting mock %s (%s port %s)...' % (name, get_service_protocol(), port))
    if config.USE_SSL and not backend_port:
        backend_port = get_free_tcp_port()
    if backend_port:
        start_proxy_for_service(key, port, backend_port, update_listener)
    if config.BUNDLE_API_PROCESSES:
        return multiserver.start_api_server(key, backend_port or port)
    return start_moto_server_separate(key, port, name=name, backend_port=backend_port, asynchronous=asynchronous)
def start_moto_server_separate(key, port, name=None, backend_port=None, asynchronous=False):