Refactor the image fetching interface to use anchore/stereoscope

[wagoodman]

Today we're shelling out to docker in order to get the image archive for analysis. A lot of the learnings from working on dive made its way into https://github.com/anchore/stereoscope and could replace much of the docker/podman interop code here (and would no longer be shelling out, which is ideal). I'll start tracking the related bugs to this issue here (it seems there is at least more than one!)...