Ah - networking? OK so in an attempt to do a “basic” thing, I ran systemctl restart docker.service then ran ./launcher rebuild app and (after what seemed like ten thousand years) it came back up and worked! Now says I’m running “3.3.0.beta3-dev” which sounds like a good thing, I think? It appears to be working at least.
Phew - thanks for your help! I love Discourse!