skopeo sync: Support incremental updates to dir storage

I'm try to copy a full registry with skopeo and keep it sync. but it's not possible or...at least only once.

for i in `podman search --format "{{.Name}}" $REGISTRY/`; do
	skopeo sync --scoped --all --src docker --dest dir $i $LOCAL_DIR/
done

unfortunately it's work for the first run but not in the second case. it's be very useful to be an overwrite and a delete option. in this case we can overwrite a local directory and keep updated with a given registry. of course i can write a more complicated shell script and it'll work but imho this would be a very useful feature.