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)
Fix double single dash arg issues in docs

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)
Merge pull request #4796 from cpuguy83/fix_single_dash_args_in_dcs

Fix double single dash arg issues in docs
Added Eixo::Docker to the list of libraries

Docker-DCO-1.1-Signed-off-by: Javier Gómez <alambike@gmail.com> (github: alambike)
Merge pull request #4803 from alambike/patch-1

Added Eixo::Docker to the list of libraries
Merge pull request #4518 from tianon/sync-DCO-with-developercertificate.org

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