ci: use python:3.12 container to satisfy >=3.12

.forgejo/workflows/docs.yml

@@ -5,19 +5,22 @@ on:
       - main
 jobs:
   deploy:
-    runs-on: node:22
+    runs-on:
+      container:
+        image: python:3.12
     steps:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
 
       - run: python3 --version
-      - run: pip3 install zensical
-      - run: pip3 install -e .
 
       - run: |
           python3 -m venv /tmp/venv
           . /tmp/venv/bin/activate
+          pip install --upgrade pip
+          pip install zensical
+          pip install -e .
           zensical build --clean
 
       - name: Deploy to pages branch