NginX reverse proxy for a Jenkins in Nomad issue

I’ve moved a Jenkins instance from a dedicated node into a Nomad cluster ran as a docker, and had a very strange issue with redirections: most of the locations worked, but clicking on “/manage” would lead to “https://jenkins/manage” instead of “https://jenkins.fqdn/manage”, breaking the URL of course. I could not find a root cause in either Internet-facing load-balancer, nor in internal …