proposal: easy way to upload images as files to buildkit

[gilescope]

Use case

It's pretty trivial to turn an image into a file:

docker save alpine > alpine.tar

For docker, it's pretty trivial to load an image:

docker load < alpine.tar

But try doing the latter in Earthbuild seems tricky. There doesn't seem to be an easy / elegant way to add an image into earthly's buildkit's local image cache. Creating a local docker registry and configuring buildkit's config to load from that works but is quite a few hoops for something that should be easy.

Expected Behavior

IDK how this should look, maybe:

FROM file://image.tar