I'm experimenting with the v2 branch and ran into an issue with the basic use case below. Notice it's using the default builder (no setup-buildx-action step) with the docker driver.

jobs:
  build:
    - uses: actions/checkout@v2

    - uses: docker/login-action@v1
      with:
        registry: ${{ env.REGISTRY }}
        username: ${{ env.USER }}
        password: ${{ secrets.PASSWORD }}

    - uses: docker/build-push-action@v2-build-push
      with:
        context: .
        tags: ${{ env.REGISTRY }}/myapp:latest
        push: true

The output for the build step is:

Run docker/build-push-action@v2-build-push
📣 Buildx version: 0.4.2
🏃 Starting build...
/usr/bin/docker buildx build --tag myregistry.io/myapp:latest --iidfile /tmp/docker-build-push-Ghos6S/iidfile --file ./Dockerfile --push .
auto-push is currently not implemented for docker driver
##[error]The process '/usr/bin/docker' failed with exit code 1

I realize the error is coming from buildx upstream, but considering this action is called "build-push", I think users would expect this functionality to be implemented by the action if the underlying tool doesn't support it — or at least have a large warning in the README. My use case is more complex than this example, and part of the appeal of this action is that it seamlessly handles pushing a dynamic number of tags.