ci: migrate release workflow to zot registry

.gitea/workflows/release.yml

@@ -12,18 +12,12 @@
 #     plus install.sh / install.ps1 / the systemd unit baked in under
 #     /opt/restic-manager/dist (the read-only fallback path the server
 #     handlers use when <DataDir>/... is empty).
-#   * Pushes to this Gitea instance's container registry under
-#     <gitea-host>/<owner>/restic-manager.
+#   * Pushes to zot OCI registry (docker.dcglab.co.uk).
 #
 # Tag fan-out
 #   * tag push: :vX.Y.Z, :X.Y, :X
 #   * tag push and X >= 1: also :latest
 #   * workflow_dispatch: only :snapshot-<shortsha>; nothing else moves.
-#
-# Why no goreleaser
-#   The architecture already routes agent distribution through the
-#   server's /agent/binary endpoint. The image is the only deliverable;
-#   binary archives would just be a second source of truth.
 
 name: Release
 
@@ -34,8 +28,8 @@ on:
   workflow_dispatch:
 
 env:
-  REGISTRY: gitea.dcglab.co.uk
-  IMAGE_NAME: ${{ gitea.repository }}
+  REGISTRY: docker.dcglab.co.uk
+  IMAGE_NAME: restic-manager
 
 # Force bash as the default shell — see ci.yml header.
 defaults:
@@ -46,19 +40,23 @@ jobs:
   image:
     name: Build + push image
     runs-on: ubuntu-latest
-    container: gitea.dcglab.co.uk/steve/ci-runner-go:2026-05-08
+    container:
+      image: docker.dcglab.co.uk/ci-runner-go:2026-05-15
+      credentials:
+        username: ${{ secrets.ZOT_USERNAME }}
+        password: ${{ secrets.ZOT_PASSWORD }}
     steps:
       - uses: actions/checkout@v4
 
       - uses: docker/setup-qemu-action@v3
       - uses: docker/setup-buildx-action@v3
 
-      - name: Log in to Gitea registry
+      - name: Log in to zot registry
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
-          username: ${{ gitea.actor }}
-          password: ${{ secrets.DEV_TOKEN }}
+          username: ${{ secrets.ZOT_USERNAME }}
+          password: ${{ secrets.ZOT_PASSWORD }}
 
       - name: Compute tags + version
         id: meta