[CI] Sign into registry

.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: https://github.com/docker/login-action
+        with:
+          username: ${{ vars.REGISTRY_USER_USERNAME }}
+          username: ${{ secrets.REGISTRY_USER_PASSWORD }}
+          registry: ${{ vars.REGISTRY_BASE_URL }}
       - name: Run build
         uses: actions/latex@v1
         with: