add background image to GitLab CI

.gitlab-ci.yml

@@ -19,6 +19,7 @@ build:
   script:
     - mkdir -p public
     - cp src/logo.png public/logo.png
+    - cp src/bg.svg public/bg.svg
     - cp src/index.html public/index.html
     - cp src/style.css public/style.css
     - cp src/katex.js public/katex.js