The same thing happened to me a few years ago. I used DigitalOcean's Docker image and it had some message about UFW in motd, so I assumed it works with Docker. So I created a container with passwordless mongodb and it got wiped in a few hours.
And DO still have this in motd for newly created droplets:
Welcome to DigitalOcean's 1-Click Docker Droplet.
To keep this Droplet secure, the UFW firewall is enabled.
All ports are BLOCKED except 22 (SSH), 2375 (Docker) and 2376 (Docker).
Though it links to https://do.co/3j6j3po and it mentions ufw problem:
> Note: The default firewall for the Docker One-Click is UFW, which is a front end to iptables. However, Docker modifies iptables directly to set up communication to and from containers. This means that UFW won’t give you a full picture of the firewall settings. You can override this behavior in Docker by adding --iptables=false to the Docker daemon.