Brief update to this, I changed the mariadb and redis images in the docker-compose to use the images from zknt as well (instead of whatever is in the one on the Pixelfed github), now it still doesn't work but it's throwing mysql errors instead of pgsql ones. So that's an improvement I suppose.
Going to wipe everything and start again, but I suspect I'll end up just building it all myself at this rate.
Related question: Does anyone know if it's still possible to build the apps inside Docker? There was a commit months ago which removed the build section fron the docker-compose.yml and replaced it with an image (which apparently doesn't exist), if I re-add that is the existing Dockerfile still going to work for building? Or has that been dropped now?