Update .gitlab-ci.yml

.gitlab-ci.yml

@@ -3,6 +3,7 @@ image: debian:stretch
 build:
   script:
   - echo "deb http://deb.debian.org/debian stretch main" > /etc/apt/sources.list
+  - apt-get update
   - apt-get install pandoc python3-pip
   - pip3 install MarkdownPP
   - mkdir public