Manually write and annotate manifests for `bakery ci merge`

We should be able to manually create and push a manifest for the `bakery ci merge` command rather than using a local registry as temporary storage to shoehorn in `docker buildx imagetools create` commands. The [ORAS project](https://oras.land/) and its [Python SDK library](https://oras-project.github.io/oras-py/getting_started/user-guide.html).