Merge branch 'patch-15' into 'master'

docs: add docker-in-docker from registry example

See merge request gitlab-org/gitlab-ce!31101

doc/ci/docker/using_docker_build.md

@@ -575,6 +575,42 @@ For private and internal projects:
   docker login -u $CI_DEPLOY_USER -p $CI_DEPLOY_PASSWORD $CI_REGISTRY
   ```
 
+### Using docker-in-docker image from Container Registry
+
+If you want to use your own Docker images for docker-in-docker there are a few things you need to do in addition to the steps in the [docker-in-docker](#use-docker-in-docker-workflow-with-docker-executor) section:
+
+1. Update the `image` and `service` to point to your registry.
+1. Add a service [alias](https://docs.gitlab.com/ee/ci/yaml/#servicesalias)
+
+Below is an example of how your `.gitlab-ci.yml` should look like, assuming you have it configured with [TLS enabled](#tls-enabled):
+
+```yaml
+ build:
+   image: $CI_REGISTRY/group/project/docker:19.03.1
+   services:
+     - name: $CI_REGISTRY/group/project/docker:19.03.1-dind
+       alias: docker
+   variables:
+     # Specify to Docker where to create the certificates, Docker will
+     # create them automatically on boot, and will create
+     # `/certs/client` that will be shared between the service and
+     # build container.
+     DOCKER_TLS_CERTDIR: "/certs"
+     DOCKER_DRIVER: overlay2
+   stage: build
+   script:
+     - docker build -t my-docker-image .
+     - docker run my-docker-image /script/to/run/tests
+```
+
+If you forget to set the service alias the `docker:19.03.1` image won't find the 
+`dind` service, and an error like the following is thrown:
+
+```sh
+$ docker info
+error during connect: Get http://docker:2376/v1.39/info: dial tcp: lookup docker on 192.168.0.1:53: no such host
+```
+
 ### Container Registry examples
 
 If you're using docker-in-docker on your Runners, this is how your `.gitlab-ci.yml`