feat: Gitea Actions CI + env file + Gitea registry image

- .gitea/workflows/docker.yml: builds and pushes image to
  git.albert-zangerl.com/al/logwatch on every push to main
  and on version tags (v*)
- stack.env.example: template for environment variables
- stack.env: gitignored, lives on docker.lan next to the stack
- portainer-stack.yml: updated to use Gitea registry image
  and env_file reference

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/docker.yml

@@ -0,0 +1,40 @@
+name: Build & Push Docker Image
+
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - "v*"
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Log in to Gitea Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: git.albert-zangerl.com
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.GITEA_TOKEN }}
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: git.albert-zangerl.com/al/logwatch
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}