Add .NET 8.0 image

I was thinking we needed to update our .NET apps to .NET 8 since .NET 6 is out of support. We'll need to update the VS build tools on the runner to the latest version (I think we can just run Visual Studio Installer and choose Update to do it). That'll get the projects using the old CI/CD way of building. This docker image should work for the more recent projects (each project will need to change to specify the .NET version and update their CI/CD to use the new image).

I haven't tested this image locally or anything, I pretty much just assumed that if we used the 8.0 image from Microsoft's registry that all the same things would work on it.

Thoughts?