Added note about ECS CI template

Added note in documentation and in Deploy ECS template.

changelogs/unreleased/218165-add-note-no-extend-ecs.yml

+---
+title: Add note to ECS CI template
+merge_request: 32597
+author:
+type: added

doc/ci/cloud_deployment/index.md

@@ -123,6 +123,17 @@ After you're all set up on AWS ECS, follow these steps:
    task definition, making the cluster pull the newest version of your
    application.
 
+CAUTION: **Warning:**
+The [`Deploy-ECS.gitlab-ci.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Deploy-ECS.gitlab-ci.yml)
+template includes both the [`Jobs/Build.gitlab-ci.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Jobs/Build.gitlab-ci.yml)
+and [`Jobs/Deploy/ECS.gitlab-ci.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Jobs/Deploy/ECS.gitlab-ci.yml)
+"sub-templates". Do not include these "sub-templates" on their own, and only include the main
+`Deploy-ECS.gitlab-ci.yml` template. The "sub-templates" are designed to only be
+used along with the main template. They may move or change unexpectedly causing your
+pipeline to fail if you didn't include the main template. Also, the job names within
+these templates may change. Do not override these jobs names in your own pipeline,
+as the override will stop working when the name changes.
+
 Alternatively, if you don't wish to use the `Deploy-ECS.gitlab-ci.yml` template
 to deploy to AWS ECS, you can always use our
 `aws-base` Docker image to run your own [AWS CLI commands for ECS](https://docs.aws.amazon.com/cli/latest/reference/ecs/index.html#cli-aws-ecs).

doc/topics/autodevops/index.md

@@ -232,6 +232,15 @@ NOTE: **Note:**
 If you have both a valid `AUTO_DEVOPS_PLATFORM_TARGET` variable and a Kubernetes cluster tied to your project,
 only the deployment to Kubernetes will run.
 
+CAUTION: **Warning:**
+Setting the `AUTO_DEVOPS_PLATFORM_TARGET` variable to `ECS` will trigger jobs
+defined in the [`Jobs/Deploy/ECS.gitlab-ci.yml` template](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Jobs/Deploy/ECS.gitlab-ci.yml).
+However, it is not recommended to [include](../../ci/yaml/README.md#includetemplate)
+it on its own. This template is designed to be used with Auto DevOps only. It may change
+unexpectedly causing your pipeline to fail if included on its own. Also, the job
+names within this template may also change. Do not override these jobs names in your
+own pipeline, as the override will stop working when the name changes.
+
 ## Auto DevOps base domain
 
 The Auto DevOps base domain is required to use

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

+# WARNING (post-GitLab 13.0):
+#
+# This CI template should NOT be included in your own CI configuration files:
+# 'review_ecs' and 'production_ecs' are two temporary names given to the jobs below.
+#
+# Should this template be included in your CI configuration, the upcoming name changes could
+# then result in potentially breaking your future pipelines.
+#
+# More about including CI templates: https://docs.gitlab.com/ee/ci/yaml/#includetemplate
+
 .deploy_to_ecs:
   image: 'registry.gitlab.com/gitlab-org/cloud-deploy/aws-ecs:latest'
   script: