Allow using non-slim renovate image

[avorima]

I see that the image tag is hard-coded. to use the slim version. I have started to play around with postUpgradeTasks, but since they run inside the slim image there aren't any useful binaries and I have to install everything, e.g. make, patch, runtimes, etc. The normal renovatebot image does have all the binaries I need, so it would be nice if I could configure the github action to use that one instead.