Accept vanilla Linux Foundation signoffs (Closes 7368)

This allows signing off commits with `git commit -s`

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

Make the DCO regex cleaner
Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)

Closes #7368
@fredlf I assume, the `user.name` and `user.email` git configs are set. I'll update the docs.