Decrease Docker BuildKit max-parallelism for ppc64le arch

.github/workflows/docker.yml

@@ -38,6 +38,15 @@ jobs:
         with:
           fetch-depth: 0  # for correct image labeling via `git describe --tags`
       - uses: docker/setup-buildx-action@v3
+      - name: Configure `max-parallelism = 1` for Docker BuildKit
+        run: echo "[worker.oci]\nmax-parallelism = 1"
+             >> /etc/buildkitd.toml
+        if: ${{ matrix.arch == 'ppc64le' }}
+      - name: Create Docker BuildKit builder
+        run: docker buildx create --use --name container \
+                                  --driver docker-container \
+                                  --config /etc/buildkitd.toml
+        if: ${{ matrix.arch == 'ppc64le' }}
 
       - name: Detect correct Git ref for image build
         id: git