v2: Hardening Builder-GHCR against empty digests and missing contexts

[DerekRoberts]

Summary\n\nDuring the Vexilon migration, we identified two brittle points in the consolidated builder:\n1. Empty Digest Success: The action currently reports 'Success' even if no image is pushed (empty digest). We need a hard failure check.\n2. Context Over-fitting: The builder defaults to a '/' directory. We should implement a fallback check: if the directory doesn't exist, default to the root build context to support single-app repositories.