It’s not currently possible to change the runtime of a service. You have the right idea that you might want to create a new service using a Python runtime and delete the old the service using a Docker runtime.
You can technically update your Dockerfile to use Python instead, but you would need to continue managing your own environment via Docker rather than using our native Python environment, which may be more appealing. Blueprints are a similar feature to docker-compose, albeit not a 1:1 mapping.
Additionally, it would be a good idea to submit your feedback for docker-compose support.