My sense is that a lot of the container CI space just kind of assumes that every run starts from nothing or a generic upstream-supplied "stack:version" container and installs everything every time. And that's fine if your app is relatively small and the dependency footprint is, say, <1GB.

But if that's not the case (robotics, ML, gamedev, etc) or especially if you're dealing with a slow, non-parallel package manager like apt, that upfront dependency install starts to take up non-trivial time— particularly galling for a step that container tools are so well equipped to cache away.

I know depot helps a bunch with this by at least optimizing caching during build and ensuring the registry has high locality to the runner that will consume the image.