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 not is_port_open(port, http_path=http_path, expect_success=expect_success):
            raise Exception()
    return retry(check, sleep=sleep_time, retries=retries)
def get_free_tcp_port():
    tcp = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    tcp.bind(('', 0))
    addr, port = tcp.getsockname()
    tcp.close()
    return port
def get_service_protocol():
    return 'https' if config.USE_SSL else 'http'
def timestamp(time=None, format=TIMESTAMP_FORMAT):
    if not time:
        time = datetime.utcnow()
    if isinstance(time, six.integer_types + (float, )):
        time = datetime.fromtimestamp(time)
    return time.strftime(format)
def retry(function, retries=3, sleep=1, sleep_before=0, **kwargs):
    raise_error = None
    if sleep_before > 0:
        time.sleep(sleep_before)
    for i in range(0, retries + 1):
        try:
            return function(**kwargs)
        except Exception as error:
            raise_error = error
            time.sleep(sleep)
    raise raise_error
def dump_thread_info():