Can't connect to Docker container running on Droplet

I’m trying to run a docker container via docker-compose on a droplet, but finding I’m not able at all to access the container from on the droplet (with curl). I followed this guide to install Docker, plus I added docker-compose.

Here’s what my docker-compose.yml looks like:

version: "2.1"
services:
  sonarr:
    image: ghcr.io/linuxserver/sonarr
    container_name: sonarr
    environment:
      - PUID=1000
      - PGID=132
      - TZ=Europe/London
    volumes:
      - ~/docker/configs/sonarr:/config
    ports:
      - 8989:8989
    restart: unless-stopped

It starts up fine, if I open a shell on the container I can see the service is running and curl it with localhost:8989. But then from outside the container (on the droplet), curl localhost:8989 returns with ‘Connection reset by peer’

I’ve tried running this exact config on a local server also running Ubuntu 20.04 and it all works as expected, I can curl from outside the container and get the response from inside (although that’s running Docker v19 but I hope that wouldn’t affect it?). All I can conclude is it’s something to do with a firewall or iptables or something on the droplet? But I’m not trying to even access it externally, purely from on the droplet.

Any advice? Thanks!