Update docker-image action to checkout@v4

This should fix the "Node.js 12 actions are deprecated" warning in
our CI jobs.  The only differences between checkout@v2 and @v4 is
the switch to nodejs-20 and the addition of a showProgress option.

.github/workflows/docker-image.yml

@@ -12,7 +12,7 @@ jobs:
     steps:
 
     - name: Checkout code
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: Update GPG keys
       run: dnf -y update fedora-gpg-keys