Add travis script for docs (#4974)

.travis.yml

@@ -12,6 +12,13 @@ notifications:
     on_failure: always
     on_success: never
 
+before_install:
+  - curl -fsSL https://get.docker.com | sh
+  - echo '{"experimental":"enabled"}' | sudo tee /etc/docker/daemon.json
+  - mkdir -p $HOME/.docker
+  - echo '{"experimental":"enabled"}' | sudo tee $HOME/.docker/config.json
+  - sudo service docker start
+
 # New secure variables can be added using travis encrypt -r kubernetes/ingress-nginx --add K=V
 env:
   global: