File: engine/swarm/secrets.md

There are several questions and answers out there about how to pull from a private repository (using the hosts ssh key & config). A lot of them are not working, not secure or unanswered:

https://stackoverflow.com/questions/47284967/docker-secrets-and-rsa-keys
https://stackoverflow.com/questions/66168403/pull-with-ssh-in-docker-container
https://stackoverflow.com/questions/56114905/how-to-share-ssh-key-through-docker-secrets-to-access-private-github-repos

If I missunderstood something I'm sorry. But as far as I am concerned I would like more documentation about it :)