What are base images?
Every application running inside a container is built upon a foundation. This foundation is called the base image and supports everything above it. A virtual machine requires an operating system that is used by the application running inside it, and in a similar way a containerized application requires a base image.
The choice of base image is very important and the decision needs to be made with due care and attention. A good choice of base image gives your application room to grow and change as your requirements grow and change. A bad choice of base image will restrict your application and can result in costly rewriting and refactoring.
A base image is often a minimized installation of a regular server operating system, like Debian, Ubuntu or CentOS. Unused or unnecessary components are removed or not installed. This leads to an image that has minimal support requirements, a small attack surface, and is easy to test and validate. It can then be used to create intermediate images to support particular software ecosystems, for example Java.