github: Upgrade checkout@v4 to checkout@v6

checkout@v5 should be the same as checkout@v4, just running on
Node 24. checkout@v6 has got a few other minor changes that
shouldn't make any difference; opt in to newer behaviours now.

.github/workflows/docker.yml

@@ -39,7 +39,7 @@ jobs:
     needs: [prepare]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           ref: ${{inputs.ref}}
       - name: Log in to Docker Hub
@@ -65,7 +65,7 @@ jobs:
     needs: [prepare]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           ref: ${{inputs.ref}}
       - name: Download toolchain