use multiple runners for the multi-arch build

.gitea/workflows/ci.yml

@@ -94,6 +94,7 @@ jobs:
       - name: Setup docker buildx
         uses: docker/setup-buildx-action@v3
       - name: Build image
+        id: build
         uses: docker/build-push-action@v6
         with:
           tags: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
@@ -102,3 +103,8 @@ jobs:
           target: prod
           platforms: ${{ matrix.arch.platform }}
           outputs: type=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:}"