I managed to get it fixed by restoring /version.php from backup into the container version 19 and then running a manual update via docker-compose exec --user www-data nextcloud php occ -vvv upgrade
and then docker-compose exec --user www-data nextcloud php occ maintenance:mode --off
.
That got me back to a running v19.
From there I could then pull the v20 docker image and run the upgrade to v20 using the same two commands.
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