Add method to docker load image into buildkit instance

[leighmcculloch]

It doesn't appear to be possible to load a docker image into a buildkit instance for use in buildx builds.

I'm converting a workflow from docker build to docker buildx using buildkit, and I don't think it is possible to completely copy the same workflow because of a missing feature. I've looked in docs for how to do this but as far as I can tell it isn't possible.

My workflow is:

1. Build an image on one machine with docker build and save the docker image using docker save -o <image>.
2. Load the image onto a second machine with docker load -i <image>.
3. Build a second image that is dependent on the first image that was loaded in step 2 using docker build.

When I replicate my flow with buildkit this is what I do:

1. Build an image on one machine with docker buildx build ... -o type=docker,dest=/tmp/image.
2. Load the image onto a second machine with docker load -i <image>.
3. Build a second image that is dependent on the first image that was loaded in step 2 using docker buildx build ....

In step 3 of the buildkit flow the build fails because it cannot see the image that was loaded in step 2. I assume this is because loading the image with docker load loads the image into the local docker images, and not into the buildkit instances images.

I don't see a way to load images into a buildx/buildkit instance in the same way we can run docker load with regular docker today. It would be awesome if there was a way to do this.

[leighmcculloch]

I may have opened this issue in the wrong repo, so I opened docker/buildx#847.