Leverage docker cache

Would it possible to add support for using docker cache?

Especially for PR validation, it would be great if it would be possible to specify a docker image that would be used for resolving layer cache. That image name would be passed to a docker build option:

--cache-from strings      Images to consider as cache sources

Also the image should be pulled from the registry first.

[zappy-shu]

Note that though the backend image has been released I am still waiting on approval of the updated docs to release the action with the new input. Pushing to get this done ASAP.

Thank you for your contribution @appleboy

[robpc]

Can we have an example of how to use this option? Looking at the go code, it just adds the option to the build command, but that will do nothing when there is no local cache (ie a CB scenario like this). In my experience you need to pull the image first, so was hoping that this change did that as well. I did some testing with this action and was unable to get it to work which was consistent with that conclusion.

- name: Docker Build
  uses: docker/build-push-action@v1
  with:
    registry: quay.io
    repository: ${{ env.quayio_repo }}
    username: ${{ env.quayio_user }}
    password: ${{ secrets.QUAY_AUTH }}
    tags: >-
      ${{ env.docker_tag }},
      ${{ env.docker_cache_tag }}
    cache_froms: ${{ env.quayio_repo }}:${{ env.docker_cache_tag }}

Edit: Tried several variations including cache_froms: quay.io/${{ env.quayio_repo }}:${{ env.docker_cache_tag }} with no success

_Edit2: PR for utility in docker image https://github.com/docker/github-actions/pull/17_