CI: Enable multiarch docker builds

[straight-shoota]

This brings docker images for aarch64 🎉

Apparently, we cannot load multiarch images into the local docker store. So the originally intended build, test, push workflow doesn't work.
I've changed it to push the images directly from the builder, and test them afterwards. That won't stop publishing broken images, but at least we'll know that something's up. Building the images is very unlikely to fail, so I don't expect any issues there.

Updates distribution-scripts dependency to crystal-lang/distribution-scripts@cf97700

This includes the following changes:

• Support multiarch docker builds distribution-scripts#399