fix: use container image for Gitea CI to provide Node.js

The act_runner doesn't have Node.js in PATH, which is required
by actions/checkout@v4. Use catthehacker/ubuntu:act-latest container.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -9,6 +9,8 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/catthehacker/ubuntu:act-latest
 
     steps:
       - name: Checkout code