support exporting cache with docker driver

[devopsmash]

As it says here:

docker - Uses the builder that is built into the docker daemon. With this driver, the --load flag is implied by default on buildx build. However, building multi-platform images or exporting cache is not currently supported.

Currently, I have to use docker-container to support cache, but I also need the --load option to export my new image to docker images It will be awesome to have support for exporting cache with the docker driver so I'll not need the --load option to speed up my build