[TASK] Update actions/checkout in docker-test.yaml

Update actions/checkout from v4 to v6 in docker-test.yaml,
which was missed in the initial actions update commit.

Author: CybotTM

.github/workflows/docker-test.yaml

@@ -19,7 +19,7 @@ jobs:
     runs-on: "ubuntu-latest"
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v4"
+        uses: "actions/checkout@v6"
 
       - name: "Prepare action (adjust configure-guides-step)"
         ##################################################################