github: Upgrade the docker actions to the latest version

This avoids warnings about the actions targeting Node 20.

There is no easily available documentation about whether there are
any other API changes in the newer versions of these actions - in
particular the bump of build-push-action which advances by two
versions.

.github/workflows/test-system-clang.yml

@@ -8,7 +8,7 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - name: Build Docker images
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v7
         with:
           context: .
           push: false