As I've commented on your reddit thread, dokku/herokuish don't automagically upgrade themselves. Herokuish is pegged to specific buildpack versions, and you would have to manually upgrade it - or have automatic system updates for non-security releases - in order to have newer versions of the buildpacks automatically installed.
Other than the fact that you happen to be running dokku, nothing about this problem is dokku-specific, and you would have hit the same issue with heroku.