The voting-app from the docker birthday party does not work on digitalocean droplet

I tried to modify the voting-app from the docker birthday challenge to run it on digitalocean (I only had to remove the dynamic volumes for the voting-app and the result-app). I also changed ports to 8888 and 8889 but that shouldn’t be an issue.
Source for the app is on https://github.com/docker/docker-birthday-3.git

However the app does not respond on the ports I defined for it. I used tcpdump to verify that tcp traffic is reaching the droplet but also the container which runs the voting-app (installed tcpdump manually in the container).
When i perform a curl -v http://127.0.0.1:8888 on the droplet it shows the correct html (so the app works), however I cannot reach it using the external IP of the droplet?

regards,
Rick