Ok, all good. Here’s what I did incase it helps others who had an older docker.
wget -qO- https://get.docker.com/ | sh
This will update docker, or rather do the commands of adding the key and repository to the latest binaries for your system. Note: it will try to scare you with this:
Warning: the "docker" command appears to already exist on this system.
As I already had a snapshot, I went for it and it seems ok. I rerun the update for Discourse like this:
./launcher rebuild app
After that I was lower on disk apparently (after all those dist and apt updates I guess):
Status: Downloaded newer image for discourse/base:2.0.20171008
You have less than 5GB of free space on the disk where /var/lib/docker is located. You will need more space to continue
I cleared up some space with this:
After that (it’ll take a while) then a launcher rebuild app completed and we’ll all good.