Harness CD is used to deploy a Docker image from a private Cloudsmith repository to an AWS ECS cluster. The deployment process involves creating a pipeline in Harness CD, which automates the build and deployment of the Docker image. The pipeline uses the Cloudsmith API to retrieve the Docker image from the private repository and then deploys it to the AWS ECS cluster. This allows developers to automate their CI/CD pipelines and deploy their applications quickly and efficiently.