Fix multi platform CD

[bertsky]

@kba I saw you struggle with my #1239 changes in https://github.com/OCR-D/core/actions/workflows/docker-image.yml afterwards. I did not test the last changes locally (and Github itself could not be used). Sorry about that!

Here is an attempt to at least gain the ARM64 target back. I am still looking for the problem with ppc64le – it used to work the first time, but perhaps now we are hitting the same scikit-build / cmake-python / opencv-python-headless issue as with arm/v7.

To test this locally, it suffices to do:

docker buildx create --name container-builder --bootstrap --use
make docker DOCKER_BUILD="docker buildx build --progress=plain --platform linux/amd64,linux/arm64/v8" DOCKER_BASE_TAG="ghcr.io/ocr-d docker.io/ocrd"

(or play with the platform argument, or do make docker-cuda etc)