Thanks Jeff.
That is a very viable option.
I am currently experimenting https://hub.docker.com/r/dreamcat4/pipework/, will report back if I succeed.
That solution seams more “integrated” and “automated”. I just specify extra environment variable when starting a container and it should just work.
With your solution I have to manage getting and renewing the lease of all IP myself. And outgoing connection from inside the container will probably still use the host IP as source IP and not its own IP.