Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/docker-build.yml

@@ -5,7 +5,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: docker/login-action@v1