Gunicorn won't start Flask app because "Failed to parse 'app' as an attribute name or function call."

Question

I'm trying to run Flask app locally in the docker and I'm facing the following problem

app.py:

class App:
    '''Base flask app'''

    def __init__(self):
        self.app = Flask(__name__)
    
    @property
    def flask_app(self) -> Flask:
        '''Return flask app'''

        return self.app

app_instance = App()

My Dockerfile:

...
CMD exec gunicorn --bind :8080 --workers 1 --threads 8 --timeout 0 app:app_instance.app

Error:

Failed to parse 'app_instance.app' as an attribute name or function call.