Add more info about private Docker registry

doc/ci/docker/using_docker_images.md

@@ -140,17 +140,58 @@ that runner.
 
 ## Define an image from a private Docker registry
 
-Starting with GitLab Runner 0.6.0, you are able to define images located to
-private registries that could also require authentication.
-
-All you have to do is be explicit on the image definition in `.gitlab-ci.yml`.
-
-```yaml
-image: my.registry.tld:5000/namespace/image:tag
-```
-
-In the example above, GitLab Runner will look at `my.registry.tld:5000` for the
-image `namespace/image:tag`.
+> **Notes:**
+- This feature requires GitLab Runner **1.8** or higher
+- For GitLab Runner versions **>= 0.6, <1.8** there was a partial
+  support for using private registries, which required manual configuration
+  of credentials on runner's host. We recommend to upgrade your Runner to
+  at least version **1.8** if you want to use private registries.
+- If the repository is private you need to authenticate your GitLab Runner in the
+  registry. Learn more about how [GitLab Runner works in this case][runner-priv-reg].
+
+As an example, let's assume that you want to use the `registry.example.com/private/image:latest`
+image which is private and requires you to login into a private container registry.
+To configure access for `registry.example.com`, follow these steps:
+
+1. Do a `docker login` on your computer:
+
+     ```bash
+     docker login registry.example.com --username my_username --password my_password
+     ```
+
+1. Copy the content of `~/.docker/config.json`
+1. Create a [secret variable] `DOCKER_AUTH_CONFIG` with the content of the
+   Docker configuration file as the value:
+
+     ```json
+     {
+         "auths": {
+             "registry.example.com": {
+                 "auth": "bXlfdXNlcm5hbWU6bXlfcGFzc3dvcmQ="
+             }
+         }
+     }
+     ```
+
+1. Do a `docker logout` on your computer if you don't need access to the
+   registry from it:
+
+     ```bash
+     docker logout registry.example.com
+     ```
+
+1. You can now use any private image from `registry.example.com` defined in
+   `image` and/or `services` in your [`.gitlab-ci.yml` file][yaml-priv-reg]:
+
+      ```yaml
+      image: my.registry.tld:5000/namespace/image:tag
+      ```
+
+      In the example above, GitLab Runner will look at `my.registry.tld:5000` for the
+      image `namespace/image:tag`.
+
+You can add configuration for as many registries as you want, adding more
+registries to the `"auths"` hash as described above.
 
 If the repository is private you need to authenticate your GitLab Runner in the
 registry. Learn how to do that on
@@ -283,4 +324,5 @@ creation.
 [tutum/wordpress]: https://hub.docker.com/r/tutum/wordpress/
 [postgres-hub]: https://hub.docker.com/r/_/postgres/
 [mysql-hub]: https://hub.docker.com/r/_/mysql/
-[runner-priv-reg]: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/docs/configuration/advanced-configuration.md#using-a-private-docker-registry
+[runner-priv-reg]: http://docs.gitlab.com/runner/configuration/advanced-configuration.html#using-a-private-container-registry
+[secret variable]: ../variables/README.md#secret-variables