Weekly non-local-container task: localize the Tailscale operator stack on **both clusters**.
## What
- **`containers/tailscale-operator/`** (new) — builds `cmd/k8s-operator` v1.94.2 from the forge mirror, mirroring upstream's mkctr recipe (`/usr/local/bin/operator`, `ts_kube,ts_package_container` go tags, version stamps). `container.py` (dagger) for indri/arm64; `default.nix` for ringtail/amd64.
- **`containers/tailscale/container.py`** (new) — dagger/arm64 build of the proxy image (containerboot), mirroring the upstream Dockerfile (iptables-legacy symlinks, `/tailscale/run.sh` compat). Ringtail already consumes the existing nix build; this completes parity for indri.
- **Version pinned at v1.94.2** (same as currently deployed) — this PR is a pure supply-chain swap, no version change. v1.96.x is avoided deliberately (MagicDNS-in-containers regression).
- Docs-first: tailscale-operator card gains **Local Images** and **Rollout Safety** sections.
## Rollout plan (after image builds)
1. Manifest commit: per-overlay `images:` override for the operator + ProxyClass strategic-merge patch on indri (kustomize `images:` can't touch CR fields).
2. `argocd app set tailscale-operator --revision <branch> && argocd app sync` — indri first, verify, then ringtail.
3. **Shadow-device safety**: device identity lives in the tailscale state Secrets; an image swap re-uses existing node keys, so no `-1` clones. State Secrets are not touched. Post-sync verification: pod health, device names unchanged, `mise run services-check`.
## Follow-ups (not this PR)
- `dnsconfig` nameserver image (`tailscale/k8s-nameserver:stable`) still upstream.
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Reviewed-on: #374
## Summary
Adds the first cut of a local nix build for `docker.io/tailscale/tailscale` and rewires only the ringtail tailscale-operator overlay to use it. Indri's overlay continues pulling upstream — minikube on indri is being decommissioned in favor of ringtail's k3s, so investing in dual-cluster routing here would be wasted churn.
## Changes
- `containers/tailscale/default.nix` — `buildGoModule` over `cmd/tailscale`, `cmd/tailscaled`, `cmd/containerboot`; packaged via `dockerTools.buildLayeredImage` with `cacert`, `iptables` (legacy symlink to match upstream Synology compat), `iproute2`, `tzdata`, `busybox`.
- `argocd/manifests/tailscale-operator-ringtail/kustomization.yaml` — kustomize `images:` rewrite swapping `docker.io/tailscale/tailscale` → `registry.ops.eblu.me/blumeops/tailscale:v1.94.2-67af7a8-nix`.
- `docs/changelog.d/mirror-tailscale-container.infra.md` — fragment.
## Pin rationale
v1.94.2 matches `service-versions.yaml:96` and the current ProxyClass exactly — this PR is "make it local," not "upgrade tailscale." Version bumps come as follow-up C0/C1 changes once we decide to test newer (v1.96.x had a Fly-side MagicDNS regression; v1.98.0 is current upstream stable).
## Test plan
- [x] Image built successfully on ringtail nix-container-builder (run #528).
- [x] Image visible in registry: `registry.ops.eblu.me/blumeops/tailscale:v1.94.2-67af7a8-nix`.
- [ ] Deploy from branch: `argocd app set tailscale-operator-ringtail --revision mirror-tailscale-container && argocd app sync tailscale-operator-ringtail`.
- [ ] Verify proxy pods restart with new image and existing tailnet ingresses (e.g., authentik, immich, tempo) keep resolving.
- [ ] After merge: rebuild on main SHA, update kustomization, run `services-check`.
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Reviewed-on: #347
