Add kde:websites/jekyll-kde-theme to static-websites docker image

Install kde:websites/jekyll-kde-theme in the static-websites docker image. With this patch, the new konsole website should now works. See https://phabricator.kde.org/D20701.

I just tested that the docker image build with docker build, but I'm not very experimented with Docker, so I couldn't figure out how to do anything else. Could someone try it, or even better give me the commands to test it myself? ;)