Actually, updating #nextcloud to #postgresql on #alpine was pretty easy
1. php occ db:convert-type pgsql nextcloud localhost nextcloud
2. (Disable some apps until that command worked)
3. ssh $OLD_SERVER pg_dump nextcloud | psql nextcloud
4. rsync data dir onto new server
5. Mess around with some config stuff
6. Update https://wiki.alpinelinux.org/wiki/Nextcloud wiki page to be slightly more up-to-date