Decide if DockerHub should still be supported

[dideler]

Related to #1.

As of yesterday, May 1st 2021, it's been six months since the new DockerHub retention policy was introduced. That means they will start deleting stale images (6 months or older without push/pull activity) from now on.

All the images were still present today. Not sure if they gave a grace period, since I doubt all versions have been pulled in the last six months. They'll be present for at least another six months because I pulled them all today.

Decide if it's worth the maintenance hassle to continue using DockerHub in addition to GitHub Container Registry.

One benefit is that a lot of tooling defaults to DockerHub for images.

Also consider creating a GitHub action that runs very infrequently (like once every six months) to reset the counter.
Need to do docker pull --all-tags dideler/fish-shell more or less.

[dideler]

Also keep an eye on GitHub Billing: https://github.com/settings/billing

Their docs weren't super clear on how pricing works for GHCR.