docker: add traefik support

[missytake]

I pulled out traefik support from #614 so we can think about it individually, and merge normal docker support first.

[hpk42]

why traefik and not any other alternative to acme?
It may be better to make using acmetool optional, and only document how to integrate non-acmetool things, providing some hooks or so.

[Keonik1]

@hpk42
In my experience, Traefik has one of the best supports for working with Docker containers. All you need to do is specify the labels, and Traefik will do the rest.

In other alternatives (except for Kubernetes Ingress), this is much less convenient and native.