Add setup of python so that bumpversion can be installed by pip.

.github/workflows/docker.yml

@@ -11,6 +11,11 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+    - name: Set up Python
+      uses: actions/setup-python@v4
+      with:
+        python-version: "3.12"
+
     - name: Checkout code
       uses: actions/checkout@v4