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.
os.environ[LOCALSTACK_INFRA_PROCESS] = '1'
        is_in_docker = in_docker()
        
        if not is_in_docker and 'docker' in config.LAMBDA_EXECUTOR and not is_linux():
            print(('!WARNING! - Running outside of Docker with LAMBDA_EXECUTOR=%s can lead to '
                   'problems on your OS. The environment variable $LOCALSTACK_HOSTNAME may not '
                   'be properly set in your Lambdas.') % config.LAMBDA_EXECUTOR)
        
        load_plugins()
        event_publisher.fire_event(event_publisher.EVENT_START_INFRA,
            {'d': is_in_docker and 1 or 0, 'c': in_ci() and 1 or 0})
        
        setup_logging()
        
        apis = canonicalize_api_names(apis)
        
        os.environ['AWS_REGION'] = config.DEFAULT_REGION
        os.environ['ENV'] = ENV_DEV
        
        if not os.environ.get(ENV_INTERNAL_TEST_RUN):
            register_signal_handlers()
        
        check_aws_credentials()
        
        install.install_components(apis)
        
        sleep_time = 5
        
        thread = None
        if 'elasticsearch' in apis or 'es' in apis: