make docker: --build-arg BASE_IMAGE fails if BASE_IMAGE is not a single string

[kba]

 docker buildx build --progress=plain --platform linux/amd64,linux/arm64/v8 --push -f Dockerfile.cuda -t ghcr.io/ocr-d/core-cuda -t docker.io/ocrd/core-cuda --target ocrd_core_base --build-arg BASE_IMAGE=ghcr.io/ocr-d docker.io/ocrd/core  .
ERROR: "docker buildx build" requires exactly 1 argument.
See 'docker buildx build --help'.

Usage:  docker buildx build [OPTIONS] PATH | URL | -

Start a build
make: *** [Makefile:399: docker-cuda] Error 1

https://github.com/OCR-D/core/actions/runs/9962367832/job/27525950387