Thanks. I think you could try just adding 2G or 4G of swap. But the message about out of date Docker, and the running of 18.04LTS, means you’d be in a much better position if you migrate to a new instance with a fresh new OS installation: 24.04 preferably. I run both 24.04 and 22.04 at present.
Migrating to a new fresh instance is probably less effort overall than trying to upgrade in place. It might even be less disruptive. You are very likely to have a bit of downtime whatever you do.
Step 1 is to take a backup (with downloads) and download it for safe keeping. Also take a copy of your app.yml file.
I did a migration yesterday, for similar reasons. Also, I could get a better machine for the same cost by switching to a newer offering from the hosting provider.
I followed the migration recipe and tips here
Steps involved to downgrade from 2GB to 1GB on DO? - #3 by merefield
Also for best results read
MKJ’s Opinionated Discourse Deployment Configuration
You are a little short of disk space too, especially if you add swap. I usually do
du -kx / | sort -n | tail -55
or similar to see if there’s anything large that need not be there.