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.
moto_server_cmd = '%s/bin/moto_server' % LOCALSTACK_VENV_FOLDER
    if not os.path.exists(moto_server_cmd):
        moto_server_cmd = run('which moto_server').strip()
    cmd = 'VALIDATE_LAMBDA_S3=0 %s %s -p %s -H %s' % (moto_server_cmd, key, backend_port or port, constants.BIND_HOST)
    return do_run(cmd, asynchronous)
def start_local_api(name, port, method, asynchronous=False):
    print('Starting mock %s service (%s port %s)...' % (name, get_service_protocol(), port))
    if asynchronous:
        thread = FuncThread(method, port, quiet=True)
        thread.start()
        TMP_THREADS.append(thread)
        return thread
    else:
        method(port)
def stop_infra():
    if common.INFRA_STOPPED:
        return
    common.INFRA_STOPPED = True
    event_publisher.fire_event(event_publisher.EVENT_STOP_INFRA)
    generic_proxy.QUIET = True
    common.cleanup(files=True, quiet=True)
    common.cleanup_resources()
    lambda_api.cleanup()
    time.sleep(2)
    
    
def check_aws_credentials():