expose docker caching from host to the VM

It's too bad starting a dstack guest vm instance is so slow, even when the application containers mostly consist of frequently-reused containers. It would be great if the guest could use the host as an (untrusted) local cache.

Here's relevant docs on how to configure container drivers (should be applicable to podman) registries to point elsewhere (i.e. to a service on the host)
https://github.com/containers/image/blob/main/docs/containers-registries.conf.5.md#remapping-and-mirroring-registries

excuse the brief design issue :]