Middleware to add the "/" if needed

Could you try something like that:

- traefik.http.middlewares.strip-prefix.replacepathregex.regex=^(/[a-z0-9_]+)
- traefik.http.middlewares.strip-prefix.replacepathregex.replacement=${1}/

https://docs.traefik.io/v2.0/middlewares/replacepathregex/