perf: parallelize multi-arch Docker builds with native ARM runners

Replace sequential QEMU-emulated builds with parallel native builds
using a matrix strategy (ubuntu-latest for amd64, ubuntu-24.04-arm
for arm64) and manifest merging. This eliminates the ~5-10x QEMU
overhead and should cut Docker build time roughly in half.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/bump.yml

@@ -48,13 +48,20 @@ jobs:
     needs: bump
     uses: ./.github/workflows/ci.yml
 
-  release:
-    name: Build & Push Docker Image
-    runs-on: ubuntu-latest
+  build:
+    name: Build Docker (${{ matrix.platform }})
     needs: [bump, ci]
     permissions:
-      contents: write
       packages: write
+    strategy:
+      fail-fast: true
+      matrix:
+        include:
+          - platform: linux/amd64
+            runner: ubuntu-latest
+          - platform: linux/arm64
+            runner: ubuntu-24.04-arm
+    runs-on: ${{ matrix.runner }}
     steps:
       - uses: actions/checkout@v4
         with:
@@ -65,13 +72,6 @@ jobs:
         uses: docker/metadata-action@v5
         with:
           images: ghcr.io/${{ github.repository }}
-          tags: |
-            type=semver,pattern={{version}},value=${{ needs.bump.outputs.tag }}
-            type=semver,pattern={{major}}.{{minor}},value=${{ needs.bump.outputs.tag }}
-            type=raw,value=latest
-
-      - name: Set up QEMU (for multi-platform builds)
-        uses: docker/setup-qemu-action@v3
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
@@ -83,17 +83,74 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Build and push Docker image
+      - name: Build and push by digest
+        id: build
         uses: docker/build-push-action@v6
         with:
           context: .
-          platforms: linux/amd64,linux/arm64
-          push: true
-          tags: ${{ steps.meta.outputs.tags }}
+          platforms: ${{ matrix.platform }}
           labels: ${{ steps.meta.outputs.labels }}
           provenance: false
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-from: type=gha,scope=${{ matrix.platform }}
+          cache-to: type=gha,scope=${{ matrix.platform }},mode=max
+          outputs: type=image,"name=ghcr.io/${{ github.repository }}",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@v4
+        with:
+          name: digest-${{ matrix.runner }}
+          path: ${{ runner.temp }}/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  merge:
+    name: Merge manifests & release
+    runs-on: ubuntu-latest
+    needs: [bump, build]
+    permissions:
+      contents: write
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Download digests
+        uses: actions/download-artifact@v4
+        with:
+          path: ${{ runner.temp }}/digests
+          pattern: digest-*
+          merge-multiple: true
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=semver,pattern={{version}},value=${{ needs.bump.outputs.tag }}
+            type=semver,pattern={{major}}.{{minor}},value=${{ needs.bump.outputs.tag }}
+            type=raw,value=latest
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v3
+        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 $(jq -cr '.tags | map("-t " + .) | join(" ")' <<< "$DOCKER_METADATA_OUTPUT_JSON") \
+            $(printf 'ghcr.io/${{ github.repository }}@sha256:%s ' *)
 
       - name: Create GitHub Release
         uses: softprops/action-gh-release@v2
@@ -104,7 +161,7 @@ jobs:
   pin-docker-compose:
     name: Pin docker-compose to released version
     runs-on: ubuntu-latest
-    needs: [bump, release]
+    needs: [bump, merge]
     permissions:
       contents: write
     steps:

.github/workflows/release.yml

@@ -13,10 +13,18 @@ jobs:
   ci:
     uses: ./.github/workflows/ci.yml
 
-  release:
-    name: Build & Push Docker Image
-    runs-on: ubuntu-latest
+  build:
+    name: Build Docker (${{ matrix.platform }})
     needs: ci
+    strategy:
+      fail-fast: true
+      matrix:
+        include:
+          - platform: linux/amd64
+            runner: ubuntu-latest
+          - platform: linux/arm64
+            runner: ubuntu-24.04-arm
+    runs-on: ${{ matrix.runner }}
     steps:
       - uses: actions/checkout@v4
 
@@ -25,13 +33,6 @@ jobs:
         uses: docker/metadata-action@v5
         with:
           images: ghcr.io/${{ github.repository }}
-          tags: |
-            type=semver,pattern={{version}}
-            type=semver,pattern={{major}}.{{minor}}
-            type=raw,value=latest
-
-      - name: Set up QEMU (for multi-platform builds)
-        uses: docker/setup-qemu-action@v3
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
@@ -43,17 +44,71 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Build and push Docker image
+      - name: Build and push by digest
+        id: build
         uses: docker/build-push-action@v6
         with:
           context: .
-          platforms: linux/amd64,linux/arm64
-          push: true
-          tags: ${{ steps.meta.outputs.tags }}
+          platforms: ${{ matrix.platform }}
           labels: ${{ steps.meta.outputs.labels }}
           provenance: false
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-from: type=gha,scope=${{ matrix.platform }}
+          cache-to: type=gha,scope=${{ matrix.platform }},mode=max
+          outputs: type=image,"name=ghcr.io/${{ github.repository }}",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@v4
+        with:
+          name: digest-${{ matrix.runner }}
+          path: ${{ runner.temp }}/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  merge:
+    name: Merge manifests & release
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Download digests
+        uses: actions/download-artifact@v4
+        with:
+          path: ${{ runner.temp }}/digests
+          pattern: digest-*
+          merge-multiple: true
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=raw,value=latest
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v3
+        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 $(jq -cr '.tags | map("-t " + .) | join(" ")' <<< "$DOCKER_METADATA_OUTPUT_JSON") \
+            $(printf 'ghcr.io/${{ github.repository }}@sha256:%s ' *)
 
       - name: Create GitHub Release
         uses: softprops/action-gh-release@v2
@@ -63,7 +118,7 @@ jobs:
   pin-docker-compose:
     name: Pin docker-compose to released version
     runs-on: ubuntu-latest
-    needs: release
+    needs: merge
     permissions:
       contents: write
     steps: