guix pack and exposing ports

[Todor Kondić]

Hello,

Please bear with me since I am not that very docker savvy. As far as I 
understand, normally, one can expose certain ports in a docker description 
file. But, how to do that when using `guix pack -f docker` command where the 
docker recipe is hidden from the user?

Also, I note I'm getting ~10 GiG image for a flimsy program depending on R and 
shiny. Somehow that doesn't feel right. :)