ci: do always use v4 checkout action

Author: tboerger

.github/workflows/docker.yml

@@ -19,7 +19,7 @@ jobs:
     steps:
       - name: Checkout source
         id: source
-        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+        uses: actions/checkout@v4
 
       - name: Detect version
         id: version