What is wrong?

BlueOcean button points to host with different port from the one specified in
```
/etc/default/jenkins
...
HTTP_PORT=9001
...
```
See attached image.

How should it be?

BlueOcean button should point to the same host:port, which Jenkins is using.

Reproduce

Install jenkins
Install BlueOcean plugin.
Restart jenkins.
Due to default parameters, everything works fine.
Open /var/default/jenkins and change HTTP_PORT=8080 to HTTP_PORT=9000
Restart jenkins.
Everything still works fine.
Open /var/default/jenkins and change HTTP_PORT=9000 to HTTP_PORT=9001
Restart jenkins.
BlueOcean button does not change its location

Cause

Blue ocean uses the Jenkins URL setting extensively for guessing the URL to the server. Many other things, such as email links, also do this. It should be reasonable to expect that Jenkins URL could be set by the environment.