feat: allow building pongo image from specially tagged test image

[hanshuebner]

This change adds the possibility to specify a temporary test image as KONG_IMAGE, similar to using the 'dev' and 'dev-ee' pseudo versions. The image name contains the git tag from which it was built. The added logic will determine KONG_VERSION and whether we're dealing with CE or EE from the image name

[fffonion]

@hanshuebner i was working on running pongo with current commit of build and found this PR might be related. is this PR still we want to persue? if so could you describe what's left/what's not working so i can pick up?

[hanshuebner]

@fffonion If I remember right, this was pretty much working but nobody except myself tested it and it was basically forgotten. I have no plans working on it, but feel free to involve me if there is anything I can help with to get it merged.