Added AWS credentials for Docker credential helper

doc/ci/docker/using_docker_images.md

@@ -727,6 +727,9 @@ To configure access for `aws_account_id.dkr.ecr.region.amazonaws.com`, follow th
 
 1. Make sure `docker-credential-ecr-login` is available in GitLab Runner's `$PATH`.
 
+1. Have any of the following [AWS credentials setup](https://github.com/awslabs/amazon-ecr-credential-helper#aws-credentials).
+   Make sure that GitLab Runner can access the credentials.
+
 1. Make GitLab Runner use it. There are two ways to accomplish this. Either:
 
    - Create a [variable](../variables/README.md#gitlab-cicd-environment-variables)
@@ -741,6 +744,18 @@ To configure access for `aws_account_id.dkr.ecr.region.amazonaws.com`, follow th
      }
      ```
 
+     This configures docker to use the credential helper for a specific registry.
+
+     or
+
+     ```json
+     {
+       "credsStore": "ecr-login"
+     }
+     ```
+
+     This configures docker to use the credential helper for all Amazon ECR registries.
+
    - Or, if you are running self-managed Runners,
      add the above JSON to `${GITLAB_RUNNER_HOME}/.docker/config.json`.
      GitLab Runner will read this config file and will use the needed helper for this