Add explicit registry to our docker image references

[petejohanson]

Podman (used by default on Fedora) is an alternative to local docker/docker-daemon that's OOTB includes multiple configured registries that are searched.

We should move to explicit registries in our docker image selectors, e.g. docker.io/zmkfirmware/zmk-build-arm in order to make sure that users of Podman don't need to select them manually when doing things like running the VSCode docker plugin w/ podman.