Make the action to work with DOCKER_BUILDKIT enabled

[whoan]

Since docker in GitHub runners now use DOCKER_BUILDKIT by default, and this action currently relies in old docker build output, I disabled DOCKER_BUILDKIT in #135 but that broke the builds for some people.

The action needs to support DOCKER_BUILDKIT for those relying in such a behaviour, and because the legacy builder is deprecated:

DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            BuildKit is currently disabled; enable it by removing the DOCKER_BUILDKIT=0
            environment-variable.

[whoan]

As a workaround, I am thinking of saving the cache in a well known directory so adding an upload-artifact and a download-artifact action to our workflows, would make the action to work with DOCKER_BUILDKIT enabled.

Also, I was thinking of creating a new image with the layers of the cache (saved with --cache-to) and push that to a registry besides our image. Then the script would have to pull and cp the layers from that image and then import the cache (with --cache-from).

I also played a little trying to share cache between old and new docker builder but it was getting challenging.

Suggestions, comments, patience, support, and of course, pull requests, are very welcome.

[whoan]

Ended up using BuldKit's cache backends: https://docs.docker.com/build/cache/backends/. See #141