I never got the labels section working, so I just have a docker_args section that looks like this:
docker_args:
- "--network=web"
- "--expose=80"
- "-l traefik.backend=forums"
- "-l traefik.frontend.rule=Host:forums.domain.com,cf-cdn.domain.com"
- "-l traefik.docker.network=web"
- "-l traefik.port=80"
Not necessarily elegant, but it works.