You get a 502 error for a minute or two after an upgrade/reboot.
npm and nodejs on the server don’t make any difference to Discourse, as it gets all of that in the container.
If it’s still not back up then I’d do a command line upgrade
cd /var/discourse
./launcher rebuild app
Oh.
You can look at
docker logs -f app
and
/var/discourse/shared/logs/.....