Push CI images to ghcr.io

[gebner]

This pushes the CI base image to the ghcr.io container registry instead of storing it locally on the build machine.

The container registry is publicly accessible, so make docker-ci works out of the box (and I've changed it so that it always fetches the latest image). You can also use the image on the command line:

❯ podman run --pull=always -it --rm ghcr.io/fstarlang/fstar-ci-base opam --version
...
2.1.5

(Note that fstarlang needs to be lowercase here.)