Either create without SSL or use the command line install with a configuration description to match what you need (not actually sure if you can pass parameters other than those in the graphical installer).
The settings are fed directly to the Laravel database driver, so if you know how to configure Laravel you'll be able to configure Flarum. I've never done it so I'm not even sure if it's possible.
I guess you could also proxy the database through a local socket if necessary.