I had the same issue, but I just added my plain iptables rules to the DOCKER-USER table, which worked fine

I’m actually really happy with my Snapmaker U. Not only has it been rock steady at printing and quality, they actually open sourced both their firmware and slicer. And flashing a custom firmware is as easy as copying it to a USB drive and plugging it in

You can still run Illumos/OpenIndiana, driver support will be spotty though