I ended up actually doing it tonight. The way GPG keys were handled in 20.04 has been deprecated but it is just a case of reinstating the docker apt repository and upgrading.
So the process of upgrading an existing Discourse install on Ubuntu 20.04 to 22.04 is:
- Do the release upgrade in 20.04, following the standard steps
- Reboot the system to complete the upgrade
- From the Docker installation instructions, do steps 2 and 3 of
Set up the repository. sudo apt-get update && sudo apt-get upgrade