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.PORT_EC2
    return start_moto_server('ec2', port, name='EC2', asynchronous=asynchronous,
        backend_port=DEFAULT_PORT_EC2_BACKEND, update_listener=update_listener)
def set_service_status(data):
    command = data.get('command')
    service = data.get('service')
    service_ports = config.parse_service_ports()
    if command == 'start':
        existing = service_ports.get(service)
        port = DEFAULT_SERVICE_PORTS.get(service)
        if existing:
            status = get_service_status(service, port)
            if status == 'running':
                return
        key_upper = service.upper().replace('-', '_')
        port_variable = 'PORT_%s' % key_upper
        service_list = os.environ.get('SERVICES', '').strip()
        services = [e for e in re.split(r'[\s,]+', service_list) if e]
        contained = [s for s in services if s.startswith(service)]
        if not contained:
            services.append(service)
        update_config_variable(port_variable, port)
        new_service_list = ','.join(services)
        os.environ['SERVICES'] = new_service_list
        config.populate_configs()
        LOG.info('Starting service %s on port %s' % (service, port))
        SERVICE_PLUGINS[service].start(asynchronous=True)
    return {}
def get_services_status():