Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Build the Image
       run: docker build . --file Dockerfile --tag appuio/example-php-docker-helloworld:latest
     - name: Run the Container