ci: build multi-arch dashboard image on native runners (no QEMU)

.github/workflows/docker-dashboard.yml

@@ -7,17 +7,32 @@ permissions:
 on:
   workflow_dispatch: {}
 
+env:
+  IMAGE: ghcr.io/tx3-lang/dashboard-frontend
+
 jobs:
+  # Build each architecture on its own NATIVE runner (no QEMU emulation) and push
+  # it to GHCR by digest: amd64 on an amd64 runner, arm64 on an arm64 runner.
   build:
-    name: Build and push multi-arch image
-    runs-on: ubuntu-latest
+    name: Build ${{ matrix.platform }}
+    runs-on: ${{ matrix.runner }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - platform: linux/amd64
+            runner: ubuntu-latest
+          - platform: linux/arm64
+            runner: ubuntu-24.04-arm
     steps:
+      - name: Prepare platform name
+        run: |
+          platform=${{ matrix.platform }}
+          echo "PLATFORM_PAIR=${platform//\//-}" >> "$GITHUB_ENV"
+
       - name: Checkout
         uses: actions/checkout@v6
 
-      - name: Set up QEMU
-        uses: docker/setup-qemu-action@v4
-
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
 
@@ -28,10 +43,56 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Build and push
+      - name: Build and push by digest
+        id: build
         uses: docker/build-push-action@v7
         with:
           context: ./frontend
-          platforms: linux/amd64,linux/arm64
-          push: true
-          tags: ghcr.io/tx3-lang/dashboard-frontend:${{ github.sha }}
+          platforms: ${{ matrix.platform }}
+          outputs: type=image,name=${{ env.IMAGE }},push-by-digest=true,name-canonical=true,push=true
+
+      - name: Export digest
+        run: |
+          mkdir -p "${{ runner.temp }}/digests"
+          digest="${{ steps.build.outputs.digest }}"
+          touch "${{ runner.temp }}/digests/${digest#sha256:}"
+
+      - name: Upload digest
+        uses: actions/upload-artifact@v7
+        with:
+          name: digests-${{ env.PLATFORM_PAIR }}
+          path: ${{ runner.temp }}/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  # Combine the per-arch digests into one multi-arch manifest tagged with the SHA.
+  merge:
+    name: Merge multi-arch manifest
+    runs-on: ubuntu-latest
+    needs: [build]
+    steps:
+      - name: Download digests
+        uses: actions/download-artifact@v8
+        with:
+          path: ${{ runner.temp }}/digests
+          pattern: digests-*
+          merge-multiple: true
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v4
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v4
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Create manifest list and push
+        working-directory: ${{ runner.temp }}/digests
+        run: |
+          docker buildx imagetools create -t "${IMAGE}:${{ github.sha }}" \
+            $(printf "${IMAGE}@sha256:%s " *)
+
+      - name: Inspect image
+        run: docker buildx imagetools inspect "${IMAGE}:${{ github.sha }}"