Recognize newer docker versions without -ce/-ee suffix

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.

cmd/kubeadm/app/util/system/docker_validator.go

@@ -73,7 +73,7 @@ func (d *DockerValidator) validateDockerInfo(spec *DockerSpec, info types.Info)
 	if !matched {
 		// If it's of the new Docker version scheme but didn't match above, it
 		// must be a newer version than the most recently validated one.
-		ver := `\d{2}\.\d+\.\d+-[a-z]{2}`
+		ver := `\d{2}\.\d+\.\d+(?:-[a-z]{2})?`
 		r := regexp.MustCompile(ver)
 		if r.MatchString(info.ServerVersion) {
 			d.Reporter.Report(dockerConfigPrefix+"VERSION", info.ServerVersion, good)

cmd/kubeadm/app/util/system/docker_validator_test.go

@@ -81,6 +81,11 @@ func TestValidateDockerInfo(t *testing.T) {
 			err:  false,
 			warn: false,
 		},
+		{
+			info: types.Info{Driver: "driver_2", ServerVersion: "18.09.0"},
+			err:  false,
+			warn: true,
+		},
 	} {
 		warn, err := v.validateDockerInfo(spec, test.info)
 		if !test.err {