Do not add prefix of context twice when calculating base Url

[wolfs]

We have the problem that the images of the plugin is not showing in the UI. We have Jenkins runnning under the /ci prefix and configured this as Jenkins URL on the configuration page.
This change should fix the problem.

[rsandell]

Quoting @TWestling in pull #6

We have had a lot of issues regarding these images, fixing it for someone breaks it for two others, therefore, could you check:
Does this work with Jenkins behind a reverse proxy?
Does it work when Jenkins is not behind a reverse proxy?
Does it work with Jenkins installed at root context?
Does it work with Jenkins installed at a named context?

And combinations of the above.

[jenkinsadmin]

Thank you for a pull request! Please check this document for how the Jenkins project handles pull requests

[wolfs]

What combinations do you want me to check? If you compare the code from Hudson.getInstance().getRootUrl() and the original code from getFullImageUrl then you see that it is basically identical. The only difference is that the prefix it added to the url from the mailer, but the prefix should already be configured there.

[wolfs]

So I tested the following combinations:

named context	URL configured	behind reverse proxy
yes	no	no
yes	yes	no
no	no	no
no	yes	no
yes	yes	yes
no	yes	yes

For a reverse proxy I assume that the Jenkins URL always has to be configured in the configuration page. Moreover, when using a reverse proxy the prefix on the proxy has to be the same as the prefix configured on Jenkins. At least I did not find a manual describing on how to remap the prefix behind a reverse proxy and my intents failed.
I did use nginx as reverse proxy.
With the current configuration there are always two slashes between the base URL and the image path. This does not break anything, so I left it as is.

Do you want me to test some more?

[rsandell]

Well, lets see if it holds this time 🎱

[wolfs]

Would you pull a new release? If you prefer I can do that, too.

[rsandell]

It's on its way.

[wolfs]

Thanks!