Use buildkit with a max parallelism of 1.

.github/buildkitd.toml

@@ -0,0 +1,2 @@
+[worker.oci]
+max-parallelism = 1

.github/workflows/build-docker.yml

@@ -19,6 +19,7 @@ jobs:
       - name: Set up Docker BuildX
         uses: docker/setup-buildx-action@v2
         with:
+          config: .github/buildkitd.toml
           version: latest
       - name: Login to DockerHub
         uses: docker/login-action@v2