Problem: Registry login fails silently if secrets aren't configured
- Empty FORGEJO_REGISTRY_USER/TOKEN → docker login hangs/fails
- No clear error message about missing credentials
Solution: Add validation step that checks credentials exist
- Fails early with clear error if secrets missing
- Shows how to configure in repo settings
- Uses direct secret injection (not via env vars)
- Isolates docker config to /tmp/docker-config
Result: CI will fail fast with actionable error if credentials missing
