Merge branch 'docs-registry-hostname-part-of-service-hostname' into 'master'

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

See merge request !11729

doc/ci/docker/using_docker_images.md

@@ -210,6 +210,18 @@ 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 a private registry, the image name also includes a hostname and port
+of the registry. 
+
+```yaml
+services:
+- docker.example.com:5000/wordpress:latest
+```
+
+The service hostname will also include the registry 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