container.py builds arm64 (indri/Dagger); add default.nix to build the amd64
image on ringtail's nix-container-builder (Go 1.26, -tags all_providers).
external-secrets-ringtail now uses a thin overlay over the shared manifest,
overriding only the image to the -nix tag. Repoints the app at the overlay.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>