Merge branch 'registry-example' into 'master'

Update CI Docker docs

## What does this MR do?

Updates documentation with:

* instructions for bind-mounting for docker runners (fixes #17769 and #13898)
* examples of using the GitLab Container Registry in docker-based builds (fixes #17968 and #17967)
* update runner instructions for `gitlab-ci-multi-runner` (fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1337)
* some grammar fixes

## Are there points in the code the reviewer needs to double check?

The markdown in general. The runner commands. I haven't been able to verify the socket version works yet.

## Why was this MR needed?

Better documentation for users.

## What are the relevant issue numbers?
#17769, #13898, #17968, #17967 

## Screenshots (if relevant)

See merge request !4524

doc/ci/docker/using_docker_images.md

@@ -23,7 +23,7 @@ To use GitLab Runner with docker you need to register a new runner to use the
 `docker` executor:
 
 ```bash
-gitlab-runner register \
+gitlab-ci-multi-runner register \
   --url "https://gitlab.com/" \
   --registration-token "PROJECT_REGISTRATION_TOKEN" \
   --description "docker-ruby-2.1" \

doc/ci/examples/php.md

@@ -263,10 +263,10 @@ terminal execute:
 
 ```bash
 # Check using docker executor
-gitlab-runner exec docker test:app
+gitlab-ci-multi-runner exec docker test:app
 
 # Check using shell executor
-gitlab-runner exec shell test:app
+gitlab-ci-multi-runner exec shell test:app
 ```
 
 ## Example project

doc/ci/runners/README.md

@@ -63,7 +63,7 @@ instance.
 Now simply register the runner as any runner:
 
 ```
-sudo gitlab-runner register
+sudo gitlab-ci-multi-runner register
 ```
 
 Shared runners are enabled by default as of GitLab 8.2, but can be disabled with the
@@ -93,7 +93,7 @@ setup a specific runner for this project.
 To register the runner, run the command below and follow instructions:
 
 ```
-sudo gitlab-runner register
+sudo gitlab-ci-multi-runner register
 ```
 
 ###  Making an existing Shared Runner Specific

doc/container_registry/README.md

@@ -79,27 +79,8 @@ delete them.
 This feature requires GitLab 8.8 and GitLab Runner 1.2.
 
 Make sure that your GitLab Runner is configured to allow building docker images.
-You have to check the [Using Docker Build documentation](../../ci/docker/using_docker_build.md).
-
-You can use [docker:dind](https://hub.docker.com/_/docker/) to build your images,
-and this is how `.gitlab-ci.yml` should look like:
-
-```
- build_image:
-   image: docker:git
-   services:
-   - docker:dind
-   stage: build
-   script:
-     - docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN registry.example.com
-     - docker build -t registry.example.com/group/project:latest .
-     - docker push registry.example.com/group/project:latest
-```
-
-You have to use the credentials of the special `gitlab-ci-token` user with its
-password stored in `$CI_BUILD_TOKEN` in order to push to the Registry connected
-to your project. This allows you to automated building and deployment of your
-Docker images.
+You have to check the [Using Docker Build documentation](../ci/docker/using_docker_build.md).
+Then see the CI documentation on [Using the GitLab Container Registry](../ci/docker/using_docker_build.md#using-the-gitlab-container-registry).
 
 ## Limitations