I got it anyhow 
Yes I know but unfortunately our repo does not support plugins, it seems.
I should create it such as I would tag a container, like
repo.name/pluginname:tag
and push it, right?
This does not work in our private repo.
How can I tar one and restore it?
With docker save it did not work for me, as there is no “normal” image.
Apart from that, can I just copy a folder to the other device?