EDIT: Oh, but you have the “too many requests” error. I think what I’d do is type
docker login
and follow the instructions. It looks like a free authenticated version might solve your problem.
I’d recommend just getting a new one and following Move a Discourse site to another VPS with rsync. There’s little downtime and if something goes wrong, you can just delete the new droplet and try again.
But you’re fine for now, so you could just be able to
sudo apt update
sudo apt upgrade
# or if you want to just upgrade docker, this will probably do it.
sudo apt install docker-ce