The repository that contains the weekly builds for Debian now supports
Debian Jessie. You can see instructions here:
Or use the following repository in your sources.list:
deb http://repo-cockpitproject.rhcloud.com/debian/ jessie main
Unfortunately at the same time Debian Unstable has had dependency
problems and the last few releases of Cockpit did not build properly
there. Hopefully that will resolve itself soon.