Mark as public Github Container Registry

[sters]

WHAT

I introduced publishing container image to GHCR in this PR: https://github.com/cloudspannerecosystem/wrench/pull/111

And the result seems fine. Github actions said it's published. https://github.com/cloudspannerecosystem/wrench/actions/runs/10213036102/job/28257623430

But the image of ghcr.io/cloudspannerecosystem/wrench is still 404. I think it's marked as private so please change it to public.

WHY

I wanted to use wrench from the official docker image.