### What needs to get done Add support for pinning the build and build-base images in the rockcraft descriptors to an existing image diggest ### Why it needs to get done The motivation for this request is twofold: - Support repeatable builds, with all the advantages this entails - Trigger new builds when the base or build-base image changes using widely adopted mechanisms like [renovate](https://github.com/renovatebot/renovate). Right now there's [a workaround to support this](https://github.com/canonical/synapse-operator/blob/main/synapse_rock/rockcraft.yaml#L9) but since the base images are not pinned the resulting OCI image can be updated alongside any other change before the PR is merged