ci: add test job before Docker build

Run vitest before building the image so a failing test blocks the push.
build-and-push now depends on the test job passing.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitea/workflows/docker-build.yml

@@ -15,8 +15,28 @@ env:
   IMAGE_NAME: mozempk/tonemark
 
 jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: 'npm'
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run tests
+        run: npm test
+
   build-and-push:
     runs-on: ubuntu-latest
+    needs: test
 
     steps:
       - name: Checkout repository