Document how to login to Docker Hub in DinD

With the new [Docker Hub
ramtelimits](https://docs.docker.com/docker-hub/download-rate-limit/)
users need a way to login inside of the Docker in Docker workflow so
that they can have different rate limits which might be needed for large
deployments.

reference https://gitlab.com/gitlab-org/gitlab-runner/-/issues/27209

doc/ci/docker/using_docker_build.md

@@ -589,6 +589,147 @@ The configuration is picked up by the `dind` service.
       sub_path = "daemon.json"
 ```
 
+## Authenticating with registry in Docker-in-Docker
+
+When you use Docker-in-Docker, the [normal authentication
+methods](using_docker_images.html#define-an-image-from-a-private-container-registry)
+won't work because a fresh Docker daemon is started with the service.
+
+### Option 1: Run `docker login`
+
+In [`before_script`](../yaml/README.md#before_script) run `docker
+login`:
+
+```yaml
+image: docker:19.03.13
+
+variables:
+  DOCKER_TLS_CERTDIR: "/certs"
+
+services:
+  - docker:19.03.13-dind
+
+build:
+  stage: build
+  before_script:
+    - echo "$DOCKER_REGISTRY_PASS" | docker login $DOCKER_REGISTRY --username $DOCKER_REGISTRY_USER --password-stdin
+  script:
+    - docker build -t my-docker-image .
+    - docker run my-docker-image /script/to/run/tests
+```
+
+To log in to Docker Hub, leave `$DOCKER_REGISTRY`
+empty or remove it.
+
+### Option 2: Mount `~/.docker/config.json` on each job
+
+If you are an administrator for GitLab Runner, you can mount a file
+with the authentication configuration to `~/.docker/config.json`.
+Then every job that the runner picks up will be authenticated already. If you
+are using the official `docker:19.03.13` image, the home directory is
+under `/root`.
+
+If you mount the config file, any `docker` command
+that modifies the `~/.docker/config.json` (for example, `docker login`)
+fails, because the file is mounted as read-only. Do not change it from
+read-only, because other problems will occur.
+
+Here is an example of `/opt/.docker/config.json` that follows the
+[`DOCKER_AUTH_CONFIG`](using_docker_images.md#determining-your-docker_auth_config-data)
+documentation:
+
+```json
+{
+    "auths": {
+        "https://index.docker.io/v1/": {
+            "auth": "bXlfdXNlcm5hbWU6bXlfcGFzc3dvcmQ="
+        }
+    }
+}
+```
+
+#### Docker
+
+Update the [volume
+mounts](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#volumes-in-the-runnersdocker-section)
+to include the file.
+
+```toml
+[[runners]]
+  ...
+  executor = "docker"
+  [runners.docker]
+    ...
+    privileged = true
+    volumes = ["/opt/.docker/config.json:/root/.docker/config.json:ro"]
+```
+
+#### Kubernetes
+
+Create a [ConfigMap](https://kubernetes.io/docs/concepts/configuration/configmap/) with the content
+of this file. You can do this with a command like:
+
+```shell
+kubectl create configmap docker-client-config --namespace gitlab-runner --from-file /opt/.docker/config.json
+```
+
+Update the [volume
+mounts](https://docs.gitlab.com/runner/executors/kubernetes.html#using-volumes)
+to include the file.
+
+```toml
+[[runners]]
+  ...
+  executor = "kubernetes"
+  [runners.kubernetes]
+    image = "alpine:3.12"
+    privileged = true
+    [[runners.kubernetes.volumes.config_map]]
+      name = "docker-client-config"
+      mount_path = "/root/.docker/config.json"
+      # If you are running GitLab Runner 13.5
+      # or lower you can remove this
+      sub_path = "config.json"
+```
+
+### Option 3: Use `DOCKER_ATUH_CONFIG`
+
+If you already have
+[`DOCKER_AUTH_CONFIG`](using_docker_images.md#determining-your-docker_auth_config-data)
+defined, you can use the variable and save it in
+`~/.docker/config.json`.
+
+There are multiple ways to define this. For example:
+
+- Inside
+  [`pre_build_script`](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runners-section)
+  inside of the runner config file.
+- Inside [`before_script`](../yaml/README.md#before_script).
+- Inside of [`script`](../yaml/README.md#script).
+
+Below is an example of
+[`before_script`](../yaml/README.md#before_script). The same commands
+apply for any solution you implement.
+
+```yaml
+image: docker:19.03.13
+
+variables:
+  DOCKER_TLS_CERTDIR: "/certs"
+
+services:
+  - docker:19.03.13-dind
+
+build:
+  stage: build
+  before_script:
+    - mkdir -p $HOME/.docker
+    - echo $DOCKER_AUTH_CONFIG > $HOME/.docker/config.json
+  script:
+    - docker build -t my-docker-image .
+    - docker run my-docker-image /script/to/run/tests
+```
+
 ## Making Docker-in-Docker builds faster with Docker layer caching
 
 When using Docker-in-Docker, Docker downloads all layers of your image every