fix: resolve rollback images by local image id

[DeepZone]

Summary

• retain the local Docker image ID in newly created rollback checkpoints
• recover existing RC12 checkpoints by scanning local RepoDigests when the stored repository alias cannot be resolved
• use the resolved local image ID for rollback and rollback disposal

Regression

Fixes rollback failures such as No such image: linuxserver/heimdall@sha256:... when Docker retained the image under another repository alias.

Validation

• npm test
• node --check src/docker.js
• node --check src/server.js
• git diff --check

Local Docker integration execution was unavailable because the Docker CLI is not installed; GitHub Actions provides the Docker integration run.