Service hostname also include registry name, when using non Docker Hub registry.

doc/ci/docker/using_docker_images.md

@@ -173,6 +173,18 @@ When the job is run, `tutum/wordpress` will be started and you will have
 access to it from your build container under the hostnames `tutum-wordpress`
 (requires GitLab Runner v1.1.0 or newer) and `tutum__wordpress`.
 
+When using private registry, image name usually also include hostname and port
+of the registry. 
+
+```yaml
+services:
+- docker.example.com:5000/wordpress:latest
+```
+
+In this case, hostname will also include registry's hostname. Service will be
+available under hostnames `docker.example.com-wordpress` (requires GitLab Runner v1.1.0 or newer)
+and `docker.example.com__wordpress`.
+
 *Note: hostname with underscores is not RFC valid and may cause problems in 3rd party applications.*
 
 The alias hostnames for the service are made from the image name following these