Since docker 18.09, the ServerVersion field format changed: the `-ce`
or `-ee` suffix disappeared:

- docker 18.06: `18.06.1-ce`
- docker 18.09: `18.09.0`

This was not expected by the docker_validator version regexp, which
assumed newer docker versions ended with `-[a-z]{2}`.
This made the validator return an error, whereas we expect it to
return only a warning (by recognizing it as a newer but not yet
supported docker version).

This commit relax the version regexp to also recognize `18.09.0`.
The docker validator now returns a warning, as tested.