Interestingly, it looks like docker registry aliases aren't generally available to the target (e.g. inside of the cache_from/cache_to

[docker.registries.ghcr]
address = "<http://ghcr.io/robotpajamas|ghcr.io/robotpajamas>"

cache_from = kwargs.pop(
        "cache_from",
        {
            "type": "registry",
            "ref": f"@ghcr/{REPO_NAME}/{name}/build-cache:main",
        },
    )
    cache_to = kwargs.pop(
        "cache_to",
        {
            "type": "registry",
            "mode": "max",
            "ref": f"@ghcr/{REPO_NAME}/{name}/build-cache:main",
        },
    )

    docker_image(  # noqa: F821
        name=f"{name}-ghcr",
        cache_from=cache_from,
        cache_to=cache_to,
        **kwargs,
    )

ERROR: failed to solve: failed to configure registry cache exporter: invalid reference format

Oh, that sounds like an oversight. Filed an issue?

Not yet, was looking into a solve 🙂