Thanks for the reply,
The chmod is a bit embarrassing... :D I can't believe I left it in.
It looks like; a docker Container has no access to the systemd. I tried it use systemctl, but it failed with the error Failed to connect to bus: No such file or directory. So I continued using Upstart, even it is deprecated.
Do you have any idea how to avoid this?
I start pufferd directly to prevent it running in the background, so the stdio is accessible through docker.
Do you know any more elegant way to start it and get the stdio?