Accept vanilla Linux Foundation signoffs

This allows signing off commits with `git commit -s`

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

Cleaned up DCO regex
Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
Update CONTRIBUTING.md and test new DCO signature in this commit

Signed-off-by: Tibor Vass <teabee89@gmail.com>