Tried a restart (not yet rebuild), didn’t help.
This is the error in production.log:
Started GET "/" for 82.44.224.199 at 2019-05-29 21:36:38 +0000
Processing by ListController#latest as HTML
Completed 500 Internal Server Error in 690ms (ActiveRecord: 0.0ms)
NoMethodError (undefined method `url' for nil:NilClass)
/var/www/discourse/plugins/poll/plugin.rb:441:in `block (3 levels) in activate!'
I notice the poll plugin has been recently changed. Any idea what might be wrong, @nbianca / @zogstrip ?
I’m currently trying a rebuild to see if this helps
Update this didn’t help. Forum still dead… ideas please anyone?! 
Update I entered the Docker container and did the following:
mv /var/www/discourse/plugins/poll /tmp
exit
./launcher restart app
This seems to have got the forum working, although polls are now not rendering properly.