Localize external-secrets on ringtail (amd64 nix build) (#368)

Follow-up to #367. That PR localized external-secrets but the Dagger build (on indri's Apple Silicon runner) only produces an **arm64** image — and external-secrets also runs on **ringtail (amd64)** via the same shared manifest. This completes the localization so both clusters run the local binary on their native arch.

## Approach (matches the kube-state-metrics dual-build pattern)
- **`containers/external-secrets/default.nix`** (new) — builds the **amd64** image on ringtail's nix-container-builder. `buildGoModule` with Go 1.26 (v2.2.0 requires ≥1.26.1; nixpkgs default is 1.25.x) and `-tags all_providers`, faithful to upstream. Same v2.2.0 source from the forge mirror.
- **`argocd/manifests/external-secrets-ringtail/`** (new) — thin kustomize overlay that reuses the shared indri manifest as a base and overrides **only** the image to the `-nix` (amd64) tag. No manifest duplication.
- **`argocd/apps/external-secrets-ringtail.yaml`** — repointed at the new overlay.

Result: indri → `v2.2.0-…` (arm64, Dagger), ringtail → `v2.2.0-…-nix` (amd64, nix).

## Build
Run #581 built both arches at the branch commit. Verified the nix image is `linux/amd64`, entrypoint = the binary, user 65534.

## Deployed from branch & verified on ringtail (k3s, amd64)
- All 3 pods rolled to the nix amd64 image, `1/1 Running` (no exec-format error → arch correct)
- Controller logs clean
- **Live secret fetch proven:** force-synced `homepage/homepage-grafana` → `refreshTime` advanced, `Ready=True`
- **All 20** ringtail ExternalSecrets remain `SecretSynced=True`

## Post-merge
The `external-secrets-ringtail` app is temporarily pointed at this branch + overlay path (apps app left on `main`, manual-sync, untouched). After merge:
```
argocd app sync apps                       # picks up the new Application path on main
argocd app set external-secrets-ringtail --revision main && argocd app sync external-secrets-ringtail
```
I'll also rebuild off `main` so both clusters land on stable main-sha tags (as done for indri in #367).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #368

containers/external-secrets/default.nix

@@ -0,0 +1,56 @@
+# Nix-built External Secrets Operator (amd64, for ringtail k3s).
+# Builds v2.2.0 from the forge mirror with all secret providers compiled in,
+# faithful to upstream's `make build` (-tags all_providers). The container.py
+# sibling builds the arm64 image for indri's minikube; this default.nix builds
+# the amd64 image on ringtail's nix-container-builder.
+{ pkgs ? import <nixpkgs> { } }:
+
+let
+  version = "2.2.0";
+
+  src = pkgs.fetchgit {
+    url = "https://forge.ops.eblu.me/mirrors/external-secrets.git";
+    rev = "v${version}";
+    hash = "sha256-eAocOAp5s4CFRrpKfQr2lf3Ji+6nQQ1A5/eTw5B7v9U=";
+  };
+
+  # external-secrets v2.2.0 requires Go >= 1.26.1; nixpkgs default go is 1.25.x.
+  external-secrets = (pkgs.buildGoModule.override { go = pkgs.go_1_26; }) {
+    inherit src version;
+    pname = "external-secrets";
+    vendorHash = "sha256-0xuBK3fjAplPLAElHvKB6d+2lDz+De/s91fV4dPZwjE=";
+
+    doCheck = false;
+
+    subPackages = [ "." ];
+
+    tags = [ "all_providers" ];
+
+    ldflags = [ "-s" "-w" ];
+
+    meta = with pkgs.lib; {
+      description = "Kubernetes operator that integrates external secret management systems";
+      homepage = "https://github.com/external-secrets/external-secrets";
+      license = licenses.asl20;
+      mainProgram = "external-secrets";
+    };
+  };
+in
+
+pkgs.dockerTools.buildLayeredImage {
+  name = "blumeops/external-secrets";
+  contents = [
+    external-secrets
+    pkgs.cacert
+    pkgs.tzdata
+  ];
+
+  config = {
+    Entrypoint = [ "${external-secrets}/bin/external-secrets" ];
+    Env = [
+      "SSL_CERT_FILE=${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt"
+      "TZDIR=${pkgs.tzdata}/share/zoneinfo"
+    ];
+    User = "65534";
+  };
+}