Update .gitlab-ci.yml

.gitlab-ci.yml

 image: docker:stable
 
 before_script:
-  - apt-get update -qq
-  - apt-get install -qq git
+  - apk-get update -qq
+  - apk-get install -qq git
   # Setup SSH deploy keys
-  - 'which ssh-agent || ( apt-get install -qq openssh-client )'
+  - 'which ssh-agent || ( apk-get install -qq openssh-client )'
   - eval $(ssh-agent -s)
   - ssh-add <(echo "$SSH_PRIVATE_KEY")
   - mkdir -p ~/.ssh