feat(ecs): ability to access tag parameter value of TagParameterContainerImage (#13340)

Allows you to use the tag elsewhere within the container definition (e.g. to inform monitoring services of the release version).

Fixes: #13202

----

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

Author: alastair-watts-avrios

packages/@aws-cdk/aws-ecs/README.md

@@ -299,6 +299,8 @@ obtained from either DockerHub or from ECR repositories, or built directly from
   image directly from a `Dockerfile` in your source directory.
 - `ecs.ContainerImage.fromDockerImageAsset(asset)`: uses an existing
   `@aws-cdk/aws-ecr-assets.DockerImageAsset` as a container image.
+- `new ecs.TagParameterContainerImage(repository)`: use the given ECR repository as the image 
+  but a CloudFormation parameter as the tag.
 
 ### Environment variables
 

packages/@aws-cdk/aws-ecs/lib/images/tag-parameter-container-image.ts

@@ -49,4 +49,20 @@ export class TagParameterContainerImage extends ContainerImage {
       },
     });
   }
+
+  /**
+   * Returns the value of the CloudFormation Parameter that represents the tag of the image
+   * in the ECR repository.
+   */
+  public get tagParameterValue(): string {
+    return cdk.Lazy.string({
+      produce: () => {
+        if (this.imageTagParameter) {
+          return this.imageTagParameter.valueAsString;
+        } else {
+          throw new Error('TagParameterContainerImage must be used in a container definition when using tagParameterValue');
+        }
+      },
+    });
+  }
 }

packages/@aws-cdk/aws-ecs/test/images/tag-parameter-container-image.test.ts

@@ -21,5 +21,21 @@ nodeunitShim({
 
       test.done();
     },
+
+    'throws an error when tagParameterValue() is used without binding the image'(test: Test) {
+      // GIVEN
+      const stack = new cdk.Stack();
+      const repository = new ecr.Repository(stack, 'Repository');
+      const tagParameterContainerImage = new ecs.TagParameterContainerImage(repository);
+      new cdk.CfnOutput(stack, 'Output', {
+        value: tagParameterContainerImage.tagParameterValue,
+      });
+
+      test.throws(() => {
+        SynthUtils.synthesize(stack);
+      }, /TagParameterContainerImage must be used in a container definition when using tagParameterValue/);
+
+      test.done();
+    },
   },
 });