Update setup-buildx-action to v4

Author: j8r

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       contents: read
     steps:
       - uses: actions/checkout@v6
-      - uses: docker/setup-buildx-action@v3
+      - uses: docker/setup-buildx-action@v4
       - name: Build image
         run: docker buildx build . -o type=oci,dest=image.tar
       - name: Push image to Caasis