Added documentation about AWS ECS

Added paragraph in existing AWS doc page.
Added image of AWS ECS cluster.

changelogs/unreleased/207962-deploy-ecs.yml

+---
+title: Add CI template to deploy to ECS
+merge_request: 26371
+author:
+type: added

doc/ci/cloud_deployment/index.md

@@ -61,3 +61,61 @@ To do so, please make sure to [push your image into your ECR
 repository](https://docs.aws.amazon.com/AmazonECR/latest/userguide/docker-push-ecr-image.html)
 before referencing it in your `.gitlab-ci.yml` file and replace the `image`
 path to point to your ECR.
+
+### Deploy your application to AWS Elastic Container Service (ECS)
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/207962) in GitLab 12.9.
+
+GitLab provides a series of [CI templates that you can include in your project](../yaml/README.md#include).
+To automate deployments of your application to your [Amazon Elastic Container Service](https://aws.amazon.com/ecs/) (AWS ECS)
+cluster, you can `include` the `Deploy-ECS.gitlab-ci.yml` template in your `.gitlab-ci.yml` file.
+
+Before getting started with this process, you need a cluster on AWS ECS, as well as related
+components, like an ECS service, ECS task definition, a database on AWS RDS, etc.
+[Read more about AWS ECS](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/Welcome.html).
+
+After you're all set up on AWS ECS, follow these steps:
+
+1. Make sure your AWS credentials are set up as environment variables for your
+   project. You can follow [the steps above](#aws) to complete this setup.
+1. Add these variables to your project's `.gitlab-ci.yml` file:
+
+   ```yaml
+   variables:
+     CI_AWS_ECS_CLUSTER: my-cluster
+     CI_AWS_ECS_SERVICE: my-service
+     CI_AWS_ECS_TASK_DEFINITION: my-task-definition
+   ```
+
+   Three variables are defined in this snippet:
+
+   - `CI_AWS_ECS_CLUSTER`: The name of your AWS ECS cluster that you're
+   targeting for your deployments.
+   - `CI_AWS_ECS_SERVICE`: The name of the targeted service tied to
+   your AWS ECS cluster.
+   - `CI_AWS_ECS_TASK_DEFINITION`: The name of the task definition tied
+   to the service mentioned above.
+
+   You can find these names after selecting the targeted cluster on your [AWS ECS dashboard](https://console.aws.amazon.com/ecs/home):
+
+   ![AWS ECS dashboard](../img/ecs_dashboard_v12_9.png)
+
+1. Include this template in `.gitlab-ci.yml`:
+
+   ```yaml
+   include:
+     - template: Deploy-ECS.gitlab-ci.yml
+   ```
+
+   The `Deploy-ECS` template ships with GitLab and is available [on
+   GitLab.com](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Deploy-ECS.gitlab-ci.yml).
+
+1. Commit and push your updated `.gitlab-ci.yml` to your project's repository, and you're done!
+
+   Your application Docker image will be rebuilt and pushed to the GitLab registry.
+   Then the targeted task definition will be updated with the location of the new
+   Docker image, and a new revision will be created in ECS as result.
+
+   Finally, your AWS ECS service will be updated with the new revision of the
+   task definition, making the cluster pull the newest version of your
+   application.

lib/gitlab/ci/templates/Deploy-ECS.gitlab-ci.yml

+stages:
+  - build
+  - test
+  - review
+  - deploy
+  - production
+
+include:
+  - template: Jobs/Build.gitlab-ci.yml
+
+.deploy_to_ecs:
+  image: registry.gitlab.com/gitlab-org/cloud-deploy:latest
+  script:
+    - ecs update-task-definition
+
+review:
+  extends: .deploy_to_ecs
+  stage: review
+  environment:
+    name: review/$CI_COMMIT_REF_NAME
+  only:
+    refs:
+      - branches
+      - tags
+  except:
+    refs:
+      - master
+
+production:
+  extends: .deploy_to_ecs
+  stage: production
+  environment:
+    name: production
+  only:
+    refs:
+      - master