Build and publish a Nix-based container image for Authentik to the local registry.
## Context
Discovered while attempting [[deploy-authentik]]: the deployment references `registry.ops.eblu.me/blumeops/authentik:v1.0.0-nix` which doesn't exist. Authentik's nixpkgs package (`pkgs.authentik`) provides the `ak` wrapper which orchestrates a Go server binary and Python Django worker.
1. Verify `containers/authentik/default.nix` builds — locally via Dagger (`dagger call build-nix --src=. --container-name=authentik`) or on ringtail (the CI nix builder runs there)