ci: use node:20-alpine container to avoid Node.js setup time

.gitea/workflows/test.yml

@@ -8,19 +8,20 @@ on:
 jobs:
   unit-tests:
     runs-on: ubuntu-latest
+    container:
+      image: node:20-alpine
+      options: --user root
     # Skip if this is an auto-generated version bump commit (handled by release workflow)
     if: "!contains(github.event.head_commit.message, 'chore: bump version')"
 
     steps:
+      - name: Install system dependencies
+        run: |
+          apk add --no-cache bash git
+
       - name: Checkout code
         uses: actions/checkout@v4
 
-      - name: Setup Node.js
-        uses: actions/setup-node@v4
-        with:
-          node-version: '20'
-          cache: 'npm'
-
       - name: Install dependencies
         run: npm ci