gha: update buildkit to v0.21

Attempt to address GHA Cache API v1 deprecation causing docker image
build failures.

Refs:
* https://github.blog/changelog/2025-03-20-notification-of-upcoming-breaking-changes-in-github-actions/#decommissioned-cache-service-brownouts
* https://docs.docker.com/build/ci/github-actions/cache/#cache-backend-api

Author: sbocinec

.github/workflows/docker-image.yml

@@ -26,7 +26,7 @@ jobs:
       - uses: docker/setup-buildx-action@v3
         with:
           driver-opts: |
-            image=moby/buildkit:v0.15.2
+            image=moby/buildkit:v0.21.1
             network=host
       - name: Set build date
         run: |