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_STS
    return start_moto_server('sts', port, name='STS', asynchronous=asynchronous)
def start_redshift(port=None, asynchronous=False):
    port = port or config.PORT_REDSHIFT
    return start_moto_server('redshift', port, name='Redshift', asynchronous=asynchronous)
def start_route53(port=None, asynchronous=False):
    port = port or config.PORT_ROUTE53
    return start_moto_server('route53', port, name='Route53', asynchronous=asynchronous)
def start_ses(port=None, asynchronous=False):
    port = port or config.PORT_SES
    return start_moto_server('ses', port, name='SES', asynchronous=asynchronous)
def start_elasticsearch_service(port=None, asynchronous=False):
    port = port or config.PORT_ES
    return start_local_api('ES', port, method=es_api.serve, asynchronous=asynchronous)
def start_firehose(port=None, asynchronous=False):
    port = port or config.PORT_FIREHOSE
    return start_local_api('Firehose', port, method=firehose_api.serve, asynchronous=asynchronous)
def start_dynamodbstreams(port=None, asynchronous=False):
    port = port or config.PORT_DYNAMODBSTREAMS
    return start_local_api('DynamoDB Streams', port, method=dynamodbstreams_api.serve, asynchronous=asynchronous)
def start_lambda(port=None, asynchronous=False):
    port = port or config.PORT_LAMBDA
    return start_local_api('Lambda', port, method=lambda_api.serve, asynchronous=asynchronous)
def start_ssm(port=None, asynchronous=False):
    port = port or config.PORT_SSM
    return start_moto_server('ssm', port, name='SSM', asynchronous=asynchronous)
def start_secretsmanager(port=None, asynchronous=False):
    port = port or config.PORT_SECRETSMANAGER
    return start_moto_server('secretsmanager', port, name='Secrets Manager', asynchronous=asynchronous)
def start_ec2(port=None, asynchronous=False, update_listener=None):