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

…ant to be used in CodePipeline

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

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 their Docker files and the CDK code residing 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: