add caching workflow

.forgejo/workflows/cache.yaml

@@ -0,0 +1,24 @@
+name: cache-build
+
+on:
+  workflow_dispatch: # allows manual triggering
+  push:
+    branches:
+      # - main
+
+jobs:
+  deploy:
+    runs-on: native
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up SSH key to access age secrets
+        run: |
+          env
+          mkdir -p ~/.ssh
+          echo "${{ secrets.CD_SSH_KEY }}" > ~/.ssh/id_ed25519
+          chmod 600 ~/.ssh/id_ed25519
+
+      - name: Deploy
+      - run: attic login fediversity http://attic.fediversity.net:8080 ${{ secrets.ATTIC_PUSH_KEY }} && attic use demo && attic push demo $(nix-build)