feat: Gitea-Actions-Workflows fuer Backend- und Frontend-Images

Baut und pusht bei Push auf main sowie bei Git-Tags (vX.Y.Z) Docker-Images
fuer Backend und Frontend in die Gitea-eigene Container-Registry, zum
Deployen ins Kubernetes-Cluster. Tags: latest (main), Kurz-SHA (immer),
Versions-Tag (bei v*.*.*-Tags).

.gitea/workflows/backend-image.yml

@@ -0,0 +1,41 @@
+name: Backend Image
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*.*.*"]
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Log in to registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ vars.REGISTRY }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
+
+      - name: Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ vars.REGISTRY }}/${{ github.repository_owner }}/apicore-backend
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=sha,format=short
+            type=semver,pattern={{version}}
+
+      - name: Set up Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: ./ApiCore-Backend
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}