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 not methods:
            methods = ['GET', 'POST']
        if not path:
            path = '/'
        resources = {}
        resource_path = path.replace('/', '')
        resources[resource_path] = []
        for method in methods:
            resources[resource_path].append({
                'httpMethod': method,
                'integrations': [{
                    'type': 'HTTP',
                    'uri': target_url
                }]
            })
        return aws_stack.create_api_gateway(
            name=gateway_name,
            resources=resources,
            stage_name=self.TEST_STAGE_NAME
        )
    def connect_api_gateway_to_http_with_lambda_proxy(self, gateway_name, target_uri, methods=[], path=None):
        if not methods:
            methods = ['GET', 'POST']
        if not path: