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.
sleep_time = max(sleep_time, 10)
        
        for name, plugin in SERVICE_PLUGINS.items():
            if name in apis:
                t1 = plugin.start(asynchronous=True)
                thread = thread or t1
        time.sleep(sleep_time)
        
        check_infra(apis=apis)
        
        restore_persisted_data(apis=apis)
        print('Ready.')
        sys.stdout.flush()
        if not asynchronous and thread:
            
            
            
            while True:
                time.sleep(1)
        return thread
    except KeyboardInterrupt:
        print('Shutdown')
    except Exception as e:
        print('Error starting infrastructure: %s %s' % (e, traceback.format_exc()))
        sys.stdout.flush()
        raise e
    finally:
        if not asynchronous:
            stop_infra()