fix(docker): prefix compose image names with localhost to prevent Docker Hub lookups

randall/frontend and randall/backend were being resolved as docker.io
paths, causing unwanted registry auth attempts. The localhost/ prefix
marks them as local-only images. Also restores docker/setup-buildx-action
with the docker driver, and reverts the ECR Public mirror workaround
since it was never the root cause.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -51,6 +51,11 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+        with:
+          driver: docker
+
       - name: Build images
         run: docker compose -f cicd/docker/docker-compose.yml build