Add Forgejo action to build and push images to Attic

.forgejo/workflows/cache.yaml

@@ -0,0 +1,23 @@
+on: [ push ]
+jobs:
+  push:
+    runs-on: docker
+    container:
+      image: git.kun.is/home/forgejo-nix-action:17bb63e3d5ba294e0c11b2868e41243de1ca803f
+    steps:
+      - name: Clone repository
+        run: git clone https://git.kun.is/home/container-images.git checkout
+      - name: Setup Nix cache
+        run: |
+          /bin/attic login local https://attic.kun.is ${{ secrets.ATTIC_TOKEN }}
+          /bin/attic use home
+      - name: Build Nix derivations
+        run: |
+          nix build --extra-experimental-features nix-command --extra-experimental-features flakes --out-link blog-pim ./checkout#packages.x86_64-linux.blog-pim
+          nix build --extra-experimental-features nix-command --extra-experimental-features flakes --no-allow-dirty --out-link forgejo-nix-action ./checkout#packages.x86_64-linux.forgejo-nix-action
+          nix build --extra-experimental-features nix-command --extra-experimental-features flakes --no-allow-dirty --out-link atticd ./checkout#packages.x86_64-linux.atticd
+      - name: Push Nix derivations to cache
+        run: |
+          /bin/attic push home blog-pim
+          /bin/attic push home forgejo-nix-action
+          /bin/attic push home atticd