Split multi-arch build across dedicated runners

- ARM64 builds run on Raspberry Pi (linux-arm64 runner)
- AMD64 builds run on laptop (linux-amd64 runner)
- Builds run in parallel on separate machines
- Manifest merge creates final multi-arch image

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitea/workflows/build.yml

@@ -43,7 +43,14 @@ jobs:
   build:
     needs: check-version
     if: needs.check-version.outputs.should_build == 'true' || github.event_name == 'push' || github.event_name == 'workflow_dispatch'
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include:
+          - platform: linux/amd64
+            runner: linux-amd64
+          - platform: linux/arm64
+            runner: linux-arm64
+    runs-on: ${{ matrix.runner }}
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -58,17 +65,58 @@ jobs:
           username: ${{ secrets.REGISTRY_USERNAME }}
           password: ${{ secrets.REGISTRY_PASSWORD }}
 
-      - name: Build and push
+      - name: Build and push by digest
+        id: build
         uses: docker/build-push-action@v5
         with:
           context: .
-          platforms: linux/amd64,linux/arm64
-          push: true
-          tags: |
-            git.nicola.sh/public/nicotine-less:latest
-            git.nicola.sh/public/nicotine-less:${{ needs.check-version.outputs.latest_version }}
-          cache-from: type=registry,ref=git.nicola.sh/public/nicotine-less:buildcache
-          cache-to: type=registry,ref=git.nicola.sh/public/nicotine-less:buildcache,mode=max
+          platforms: ${{ matrix.platform }}
+          outputs: type=image,name=git.nicola.sh/public/nicotine-less,push-by-digest=true,name-canonical=true,push=true
+          cache-from: type=registry,ref=git.nicola.sh/public/nicotine-less:buildcache-${{ matrix.platform }}
+          cache-to: type=registry,ref=git.nicola.sh/public/nicotine-less:buildcache-${{ matrix.platform }},mode=max
+
+      - name: Export digest
+        run: |
+          mkdir -p /tmp/digests
+          digest="${{ steps.build.outputs.digest }}"
+          touch "/tmp/digests/${digest#sha256:}"
+
+      - name: Upload digest
+        uses: actions/upload-artifact@v4
+        with:
+          name: digests-${{ matrix.platform }}
+          path: /tmp/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  merge:
+    needs: build
+    runs-on: linux-amd64
+    steps:
+      - name: Download digests
+        uses: actions/download-artifact@v4
+        with:
+          path: /tmp/digests
+          pattern: digests-*
+          merge-multiple: true
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Gitea Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: git.nicola.sh
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
+
+      - name: Create manifest list and push
+        working-directory: /tmp/digests
+        run: |
+          docker buildx imagetools create \
+            -t git.nicola.sh/public/nicotine-less:latest \
+            -t git.nicola.sh/public/nicotine-less:${{ needs.check-version.outputs.latest_version }} \
+            $(printf 'git.nicola.sh/public/nicotine-less@sha256:%s ' *)
 
       - name: Create release
         if: needs.check-version.outputs.should_build == 'true'