feat(ecs): introduce a new Image type, TagParameterContainerImage, to…

… be used in CodePipeline (aws#11795)

While CDK Pipelines is the idiomatic way of deploying ECS applications in CDK,
it does not handle the case where the application's source code is kept in a separate source code repository from the CDK infrastructure code.
This adds a new class to the ECS module,
`TagParameterContainerImage`, that allows deploying a service managed that way through CodePipeline.

Related to aws#1237
Related to aws#7746

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-codepipeline-actions/README.md

@@ -515,7 +515,7 @@ for more details about using CloudFormation in CodePipeline.
 
 #### Actions defined by this package
 
-This package defines the following actions:
+This package contains the following CloudFormation actions:
 
 * **CloudFormationCreateUpdateStackAction** - Deploy a CloudFormation template directly from the pipeline. The indicated stack is created,
  or updated if it already exists. If the stack is in a failure state, deployment will fail (unless `replaceOnFailure`
@@ -657,6 +657,18 @@ const deployStage = pipeline.addStage({
 
 [image definition file]: https://docs.aws.amazon.com/codepipeline/latest/userguide/pipelines-create.html#pipelines-create-image-definitions
 
+#### Deploying ECS applications stored in a separate source code repository
+
+The idiomatic CDK way of deploying an ECS application is to have your Dockerfiles and your CDK code in the same source code repository,
+leveraging [Docker Assets])(https://docs.aws.amazon.com/cdk/latest/guide/assets.html#assets_types_docker),
+and use the [CDK Pipelines module](https://docs.aws.amazon.com/cdk/api/latest/docs/pipelines-readme.html).
+
+However, if you want to deploy a Docker application whose source code is kept in a separate version control repository than the CDK code,
+you can use the `TagParameterContainerImage` class from the ECS module.
+Here's an example:
+
+[example ECS pipeline for an application in a separate source code repository](test/integ.pipeline-ecs-separate-source.lit.ts)
+
 ### AWS S3 Deployment
 
 To use an S3 Bucket as a deployment target in CodePipeline: