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.
port = port or config.parse_service_ports().get(service)
    status = 'disabled' if (port or 0) <= 0 else 'running' if is_port_open(port) else 'stopped'
    return status
def restore_persisted_data(apis):
    for api in apis:
        persistence.restore_persisted_data(api)
def register_signal_handlers():
    global SIGNAL_HANDLERS_SETUP
    if SIGNAL_HANDLERS_SETUP:
        return
    
    def signal_handler(signal, frame):
        stop_infra()
        os._exit(0)
    signal.signal(signal.SIGTERM, signal_handler)
    signal.signal(signal.SIGINT, signal_handler)
    SIGNAL_HANDLERS_SETUP = True
def do_run(cmd, asynchronous, print_output=False, env_vars={}):
    sys.stdout.flush()
    if asynchronous:
        if is_debug():
            print_output = True
        outfile = subprocess.PIPE if print_output else None
        t = ShellCommandThread(cmd, outfile=outfile, env_vars=env_vars)
        t.start()
        TMP_THREADS.append(t)
        return t
    return run(cmd)
def start_proxy_for_service(service_name, port, default_backend_port, update_listener, quiet=False, params={}):