Use native runners for multiplatform build

Split build across ubuntu-latest (amd64) and ubuntu-24.04-arm (arm64)
with a merge job to create the manifest list. Much faster than QEMU
cross-compilation.

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

Author: jmgrosen

.github/workflows/build.yml

@@ -9,15 +9,27 @@ env:
   IMAGE_NAME: ${{ github.repository }}
 
 jobs:
-  build-and-push:
-    runs-on: ubuntu-latest
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - platform: linux/amd64
+            runner: ubuntu-latest
+          - platform: linux/arm64
+            runner: ubuntu-24.04-arm
+    runs-on: ${{ matrix.runner }}
     permissions:
       contents: read
       packages: write
-
     steps:
       - uses: actions/checkout@v4
 
+      - name: Prepare platform pair
+        run: |
+          platform=${{ matrix.platform }}
+          echo "PLATFORM_PAIR=${platform//\//-}" >> $GITHUB_ENV
+
       - uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
@@ -28,20 +40,70 @@ jobs:
         id: meta
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
-          tags: |
-            type=sha
-            type=raw,value=latest
-
-      - uses: docker/setup-qemu-action@v3
 
       - uses: docker/setup-buildx-action@v3
 
       - uses: docker/build-push-action@v6
+        id: build
         with:
           context: .
-          platforms: linux/amd64,linux/arm64
-          push: true
-          tags: ${{ steps.meta.outputs.tags }}
+          platforms: ${{ matrix.platform }}
           labels: ${{ steps.meta.outputs.labels }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          tags: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          outputs: type=image,push-by-digest=true,name-canonical=true,push=true
+          cache-from: type=gha,scope=${{ matrix.platform }}
+          cache-to: type=gha,scope=${{ matrix.platform }},mode=max
+
+      - 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@v4
+        with:
+          name: digests-${{ env.PLATFORM_PAIR }}
+          path: ${{ runner.temp }}/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  merge:
+    runs-on: ubuntu-latest
+    needs: build
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - name: Download digests
+        uses: actions/download-artifact@v4
+        with:
+          path: ${{ runner.temp }}/digests
+          pattern: digests-*
+          merge-multiple: true
+
+      - uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: docker/setup-buildx-action@v3
+
+      - uses: docker/metadata-action@v5
+        id: meta
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=sha
+            type=raw,value=latest
+
+      - name: Create manifest list and push
+        working-directory: ${{ runner.temp }}/digests
+        run: |
+          docker buildx imagetools create $(jq -cr '.tags | map("-t " + .) | join(" ")' <<< "$DOCKER_METADATA_OUTPUT_JSON") \
+            $(printf '${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@sha256:%s ' *)
+
+      - name: Inspect image
+        run: |
+          docker buildx imagetools inspect ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest