ci: pin python:3.11 image so venv works

.forgejo/workflows/ci.yml

@@ -1,12 +1,15 @@
 name: ci
-'on': push
+on:
+  push:
+    branches: [master, main]
 jobs:
   test:
-    runs-on:
-    - self-hosted
-    - build
+    runs-on: [self-hosted, linux, docker]
+    container: python:3.11
     steps:
-    - name: checkout
-      uses: actions/checkout@v4
-    - name: script
-      run: make test
+      - name: checkout
+        uses: actions/checkout@v4
+      - name: install make
+        run: apt-get update -qq && apt-get install -y -qq --no-install-recommends make
+      - name: test
+        run: make test