Did you run the update from the command line twice? There’s a postgres upgrade in there.
But no, it will be difficult to run a very old version since, as you point out, new old discourse isn’t compatible with new images. You need to check out an old version of Discourse docker.
Why do you want old software?
If you want old software then the best thing to do is switch to the stable branch. Or just never upgrade anything. That’s another way to play it. The oldest I think I’ve seen was 6 years