workflows: Run template testing action on our own container image

This ensures correct dependencies are present.

Also bump checkout action.

.github/workflows/test_templates.yaml

@@ -5,13 +5,9 @@ on: [push, pull_request]
 jobs:
   test_templates:
     runs-on: ubuntu-latest
-    container: centos:7
+    container: ghcr.io/quattor/quattor-test-container:latest
     steps:
     - name: Checkout code
-      uses: actions/checkout@v2
-    - name: set up panc
-      run: |
-        yum -y install http://yum.quattor.org/devel/quattor-release-1-1.noarch.rpm
-        yum -y install panc which maven
+      uses: actions/checkout@v4
     - name: test templates
       run: ./test_templates.sh