The first place I’d suggest looking is in /var/log Old log files that have grown huge are often the culprit. Another option that might help you find out what is using up space is ncdu. It will visualize where the space is being used, and you can drill down into each folder.

If you are able to free up some space, you can install it with:

sudo apt-get install ncdu

I had the same issue. What I did to solve it was enter in my digital ocean server and remove the old builds generated. Like removing an simple container, like

sudo docker rm id-container OR name 

or

sudo docker container rm id-container or name

All that inside your server root@your-ip-domain