Just to inform you I recently created a tool which could help. It is not made specifically for Traefik but answers the question for other reverse proxies

https://github.com/NathanDecou/wakontainer

It sits between a reverse proxy and a container. It starts the container when needed and stops it after a certain time without any requests. The tool is able to manage as many containers as you want, and uses containers labels to discover them.

Feel free to check it out and tell me what you think !