Push minikube-custom image

[kate-goldenring]

Instead of building the custom minikube image each CI run, we should host it. That would also enable the runtime-class-manager to use it:

docker buildx build --platform linux/amd64,linux/arm64 -t ghcr.io/spinframework/minikube-custom:v0.0.46-fixed -f ./tests/Dockerfile.minikube-custom --push .

I don't have permissions to push to our GHCR but once we get the green light, we can find someone to push it