Resync the DCO text with upstream at http://developercertificate.org/

```
Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.
```

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4518 from tianon/sync-DCO-with-developercertificate.org

Resync the DCO text with upstream at http://developercertificate.org/