[CI] Add back login

.gitea/workflows/build-docs.yml

@@ -10,6 +10,12 @@ jobs:
     steps:
       - name: Set up Git repository
         uses: actions/checkout@v6
+      - name: Sign into gitea registry
+        uses: actions/docker-login@main
+        with:
+          username: ${{ vars.REGISTRY_USER_USERNAME }}
+          password: ${{ secrets.REGISTRY_USER_PASSWORD }}
+          registry: ${{ vars.REGISTRY_BASE_URL }}
       - name: Run build
         uses: actions/latex@main
         with: