You can achieve that by letting the caching take care of it for you, there's no need for two images there.
I think OP wants to not have to reinstall every dependency again when they update just one. For that I think you'd have to use multiple Gemfiles, one for slow things that rarely change and the other for anything else new.
In one you install deps, and in the other, which relies on the first, you install your app. If you change your deps, then you rebuild the first.
I'm not a Ruby dev, but this seems like it should be quite simple?