feat(ecs): expose image name in container definition (#17793)

When adding a container to a task definition it is currently impossible to retrieve
its image name because it's the container definition that is responsible for binding
the `AssetImage`.

Expose the `imageName` so that it's possible to reference it in another construct
that needs to use/depend on the asset hash. Example: a custom resource running
a Fargate task that needs to rerun if the underlying container image changes but
not necessarily when the task definition changes.

----

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

Author: jogold

Diff for: packages/@aws-cdk/aws-ecs/lib/container-definition.ts

@@ -408,6 +408,11 @@ export class ContainerDefinition extends CoreConstruct {
    */
   public readonly referencesSecretJsonField?: boolean;
 
+  /**
+   * The name of the image referenced by this container.
+   */
+  public readonly imageName: string;
+
   /**
    * The inference accelerators referenced by this container.
    */
@@ -441,6 +446,8 @@ export class ContainerDefinition extends CoreConstruct {
     this.containerName = props.containerName ?? this.node.id;
 
     this.imageConfig = props.image.bind(this, this);
+    this.imageName = this.imageConfig.imageName;
+
     if (props.logging) {
       this.logDriverConfig = props.logging.bind(this, this);
     }

Diff for: packages/@aws-cdk/aws-ecs/test/container-definition.test.ts

@@ -2005,4 +2005,30 @@ describe('container definition', () => {
     });
 
   });
+
+  test('exposes image name', () => {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const taskDefinition = new ecs.FargateTaskDefinition(stack, 'TaskDef');
+
+    // WHEN
+    const container = taskDefinition.addContainer('cont', {
+      image: ecs.ContainerImage.fromAsset(path.join(__dirname, 'demo-image')),
+    });
+
+    // THEN
+    expect(stack.resolve(container.imageName)).toEqual({
+      'Fn::Join': [
+        '',
+        [
+          { Ref: 'AWS::AccountId' },
+          '.dkr.ecr.',
+          { Ref: 'AWS::Region' },
+          '.',
+          { Ref: 'AWS::URLSuffix' },
+          '/aws-cdk/assets:baa2d6eb2a17c75424df631c8c70ff39f2d5f3bee8b9e1a109ee24ca17300540',
+        ],
+      ],
+    });
+  });
 });