Jenkins Docker Behind Reverse Proxy Adds ":80" to the URL
If it's a brand new Jenkins, you can try creating an SSH tunnel directly into the container with port 8080 and checking if the problem also appears there. That will at least allow you to check if it's Jenkins redirecting you or Nginx.

There you can also try adjusting "Jenkins URL" in "Manage Jenkins" -> "Configure System". 

Alternatively, check  jenkins_home/jenkins.model.JenkinsLocationConfiguration.xml. jenkinsUrl tag is also present there.