Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/docker-publish.yml

@@ -28,7 +28,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       # Login against a Docker registry except on PR
       # https://github.com/docker/login-action