images: add custom image which packages SOPS. update renovate and add publishing workflow

.gitea/workflows/build.yml

@@ -0,0 +1,22 @@
+name: build
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: docker/login-action@v3
+        with:
+          registry: git.studiovita.li
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
+      - uses: docker/build-push-action@v6
+        with:
+          context: images/act
+          push: true
+          tags: git.studiovita.li/studio/actions:ubuntu-latest