Pass hostPort explicitly in container definitions

Workaround hashicorp/terraform-provider-aws#3401.

Author: Luke Hoban

aws/service.ts

@@ -327,6 +327,16 @@ function computeContainerDefinitions(
             const imageOptions = computeImage(imageName, container, ports, repository);
             const portMappings = (container.ports || []).map(p => ({
                 containerPort: p.targetPort || p.port,
+                // From https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definition_parameters.html:
+                // > For task definitions that use the awsvpc network mode, you should only specify the containerPort.
+                // > The hostPort can be left blank or it must be the same value as the containerPort.
+                //
+                // However, if left blank, it will be automatically populated by AWS, potentially leading to dirty
+                // diffs even when no changes have been made. Since we are currently always using `awsvpc` mode, we
+                // go ahead and populate it with the same value as `containerPort`.
+                //
+                // See https://github.com/terraform-providers/terraform-provider-aws/issues/3401.
+                hostPort: p.targetPort || p.port,
             }));
 
             // tslint:disable-next-line:max-line-length