Container Image is an unchangeable, static file that includes executable code. so it can run on any isolated environment.

please find the difference here

Once you’ve made changes to your application and want to make them available to your users, you typically push an image to DTR, or have a CI/CD pipeline that does it for you. A team responsible for QA will notice you’ve pushed a new image, run tests, and if the image is working as expected, they’ll promote

Yes we can use AWS spot instances for Kubernetes. For more read here.

Docker Enterprise Container Cloud is a multi-cloud management solution that gives you one set of APIs and tools to deploy, manage, and observe secure-by-default, certified, batteries-included Kubernetes clusters on any infrastructure: public cloud, private cloud, or bare metal.  For more read here