Fix workflow: pin Node 20, use Buildx, upgrade to build-push-action@v6, add layer caching

Author: Gamedirection

.github/workflows/docker-publish.yml

@@ -23,6 +23,14 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
       - name: Log in to GHCR
         uses: docker/login-action@v3
         with:
@@ -49,12 +57,14 @@ jobs:
             type=semver,pattern={{version}},enable=${{ startsWith(github.ref, 'refs/tags/') }}
 
       - name: Build & push frontend
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: ./frontend
           push: true
           tags: ${{ steps.frontend-meta.outputs.tags }}
           labels: ${{ steps.frontend-meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
 
       # ── Backend ─────────────────────────────────────────────────────────────
 
@@ -69,9 +79,11 @@ jobs:
             type=semver,pattern={{version}},enable=${{ startsWith(github.ref, 'refs/tags/') }}
 
       - name: Build & push backend
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: ./backend
           push: true
           tags: ${{ steps.backend-meta.outputs.tags }}
           labels: ${{ steps.backend-meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max