Remote image pull before deploy

[ColdHeat]

We should have some way to be able to store and use saved copies of images so that we aren't always rebuilding images on deploy.

Essentially if we build once, we should be able to store that image and then reuse it.

[MilyMilo]

Perhaps we could re-use the existing image field. If it's a registry URL, then pull the image, otherwise treat it as a local path. We could also skip the pull / build process if the registry is a CTFd registry.