Update checkout action to version 4

We were getting some warnings with version 3:

Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20: actions/checkout@v3. For more information see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

.github/workflows/docker.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Set up Docker Buildx
         id: buildx