ci: add Gitea Actions workflow to build and push Docker image

On push to main (or version tag), the workflow logs into the Gitea container
registry, builds a multi-tag image (sha-<short>, branch name, 'latest' on main,
version on tag) and pushes to gitea.siegeln.net/<owner>/<repo>.

docker-compose.prod.yml now pulls from the registry by default, with
KOCHWAS_TAG env var to pin a specific build.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/docker.yml

@@ -0,0 +1,54 @@
+name: Build & Publish Docker Image
+
+on:
+  push:
+    branches: [main]
+    tags: ['v*']
+  workflow_dispatch:
+
+env:
+  REGISTRY: gitea.siegeln.net
+  IMAGE_NAME: ${{ gitea.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to Gitea container registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.GITEA_TOKEN }}
+
+      - name: Derive tags
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=ref,event=tag
+            type=sha,format=short
+            type=raw,value=latest,enable=${{ gitea.ref == 'refs/heads/main' }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          platforms: linux/amd64
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max