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 isinstance(j, str):
            j = json.loads(j)
        self.__dict__.update(j)
    def to_dict(self):
        return json.loads(self.to_json())
    @classmethod
    def from_json(cls, j):
        j = JsonObject.as_dict(j)
        result = cls()
        result.apply_json(j)
        return result
    @classmethod
    def from_json_list(cls, l):
        return [cls.from_json(j) for j in l]
    @classmethod
    def as_dict(cls, obj):
        if isinstance(obj, dict):
            return obj
        return obj.to_dict()
    def __str__(self):
        return self.to_json()
    def __repr__(self):
        return self.__str__()
class CaptureOutput(object):