RFC: cicd: use dagger for ci

[hdonnay]

Dagger looks like a runner that would allow testing CI changes before pushing them to the repository (how novel!).

Here's the incantation I needed locally to run this:

: Download and install the "dagger" tool yourself.
podman volume create dagger_cache ||:
podman run --name dagger-engine-v0.15.2 -d -v dagger_cache:/var/lib/dagger --privileged registry.dagger.io/engine:v0.15.2
export _EXPERIMENTAL_DAGGER_RUNNER_HOST=podman-container://dagger-engine-v0.15.2
dagger call test --source=https://github.com/quay/claircore # or --source=.