Include docker registry info.

[atheken]

Hi, this is a very helpful tool, thanks for making it!

I saw that you're using GitHub Workflows to publish to ghcr.io, which is great, but I had to piece together the registry url as I usually use docker hub.

After a bit of trial and error, I can see that this works:

ghcr.io/charlesabarnes/spftoolbox:master - great!

It would be helpful if:

1. You did a tagged release to v2.1.2 (or whatever), so that there's a stable version in ghcr.
2. You added a note that this can be pulled from ghcr within the README.

Again, thanks for making this, it's great!