./launcher rebuild app for old 1.6 discourse not working

i already performed the upgrade to 3.1, its now running fine.
I’m trying to bring the old forums - which were untouched - back online for development

skipping prereqs because i can’t use the launcher rebuild command without it - docker running v1.7 and v1.8 required - but perhaps this is the issue so i’ll try updating that now.
Just a bit confused because i haven’t updated these forums so i don’t understand why the rebuild isn’t working as i haven’t changed any dependencies etc.

only thing i’ve changed is the app.yml to update the hostname and tried running rebuild, which wont work without skipping prereq