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_APIGATEWAY
    return start_moto_server('apigateway', port, name='API Gateway', asynchronous=asynchronous,
        backend_port=DEFAULT_PORT_APIGATEWAY_BACKEND, update_listener=update_listener)
def start_sns(port=None, asynchronous=False, update_listener=None):
    port = port or config.PORT_SNS
    return start_moto_server('sns', port, name='SNS', asynchronous=asynchronous,
        backend_port=DEFAULT_PORT_SNS_BACKEND, update_listener=update_listener)
def start_cloudwatch(port=None, asynchronous=False):
    port = port or config.PORT_CLOUDWATCH
    return start_moto_server('cloudwatch', port, name='CloudWatch', asynchronous=asynchronous)
def start_cloudwatch_logs(port=None, asynchronous=False):
    port = port or config.PORT_LOGS
    return start_moto_server('logs', port, name='CloudWatch Logs', asynchronous=asynchronous)
def start_events(port=None, asynchronous=False):
    port = port or config.PORT_EVENTS
    return start_moto_server('events', port, name='CloudWatch Events', asynchronous=asynchronous)
def start_sts(port=None, asynchronous=False):