Add dagger to nix-container-builder runner (#234)

## Summary
- Add `dagger` to `hostPackages` for the ringtail nix-container-builder runner
- Needed for `dagger call nix-version` fallback in the nix build workflow (authentik)
- `hostPackages` is scoped to the runner's systemd unit PATH, not system-wide
- Marks `install-dagger-on-nix-runner` Mikado card complete

## Deployment and Testing
- [ ] Merge, then `mise run provision-ringtail`
- [ ] `mise run container-build-and-release authentik` to verify nix build succeeds

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/234

docs/how-to/zot/install-dagger-on-nix-runner.md

@@ -1,7 +1,6 @@
 ---
 title: Install Dagger on Nix Runner
 modified: 2026-02-20
-status: active
 tags:
   - how-to
   - ci