try running job in a ruby container

.gitea/workflows/validate-on-push.yml

@@ -4,14 +4,12 @@ on: [push, pull_request]
 jobs:
   validate:
     runs-on: ubuntu-latest
+    container:
+      image: ruby:3.1.4
     outputs:
       completion_pct: ${{ steps.run_koans.outputs.completion_pct }}
     steps:
       - uses: actions/checkout@v4
-      - uses: ruby/setup-ruby@v1
-        with:
-          ruby-version: '3.1.4'
-          bundler-cache: true
       - id: run_koans
         run: |
           bundle exec rake | tee output.txt