Multi-builds so we can target .net 6/7/8 simultaneously

[SeanKilleen]

We currently pull from .net 7, which was updated from 6. We could probably do this and then add an additional label.

However, I'm not sure how the versioning works with that. I know how to build the container and the labels, but when we release the GitHub action, I guess we'd have to have a default version and then I'd need to see how to look into passing an argument that would work with the action. All doable I think, I just haven't done it before yet.