Split representation for selected nodes

[mikepurvis]

Originally https://github.com/patrickhoefler/dockerfilegraph/issues/682#issuecomment-3070138196

It could be useful to have other nodes able to adopt the "separated" model— common inputs would be a good fit for this (eg ubuntu:*), as would stages that collect outputs, logs, test results, etc. A possibly helpful analogue is ground planes and voltage rails on a circuit diagram, where the point is more that a thing is connected to ground, but there's no value in showing all the individual ground connections brought together.