ci: Publish to GHCR, update actions - #52

I see that the CI has failed, but I wonder if / how that is because of anything that I changed?

Ah I think I see it, there's a breaking change in the anchore scan-action v3: "Change default behavior so action fails on medium (and higher) severities"

Indeed, I reran this PR's build and it's failing at a medium severity. https://github.com/mendhak/docker-http-https-echo/actions/runs/4435829661/jobs/7795945655#step:9:34

There's an argument for the action, severity-cutoff can be set to high, or we can go back to v2. I can investigate the medium severity later.

I think that's it, working now.
I created a test tag ghcr and the publish ran just fine.

https://github.com/mendhak/docker-http-https-echo/pkgs/container/http-https-echo

docker run -p 8080:8080 -p 8443:8443 --rm -t ghcr.io/mendhak/http-https-echo:latest

I'll update the README CHANGELOG etc in a bit. And some cleaning up to do. No idea how I did this, must have been some experimenting.