Missing the capability to only expose a port

[Miaxos]

Context:

I got an automated nginx_proxy (https://github.com/jwilder/nginx-proxy) that'll detect automatically if a container is added to his network and for that, you only need to expose ports of the container with a docker run --expose 5000 something and if multiples, specify it with env variables

But with shiplift, it seems we can't only expose a port, we have to map it.

And as the function inside shiplift is called 'expose' to publish ports, I propose to do a function called 'publish' to expose a port (Avoid breaking changes?)

Links:

Well linked to #161 but for only exposing ports.

I did a proposal to add this feature because I used it in a personal project. #198