Containerd content store is not accessible on macOS

Image loading via `docker load` is slow (not incremental), so we recommend to users to make the containerd socket available to the image loader.
This way, the containerd content store can be used (if available).
On macOS, the containerd socket lives inside a Linux virtual machine, so it's not easily accessible from the host.

Docker is working on a feature to expose the content store directly on the docker socket, which will solve this problem on macOS at some point.
Meanwhile, I suggest either sticking to the (less performant) docker load fallback or using workarounds to proxy access to the containerd socket.

No repository files, tests, or entry points are named. Start by checking how image loading discovers the containerd socket on macOS and whether Docker's planned content-store exposure has landed. Done would require a defined supported path rather than the current fallback or external-socket workarounds.

Written by the indexing model from the issue text.