What file type is used to define a Docker image?

The correct answer, Dockerfile, is essential in the creation and configuration of a Docker image. A Dockerfile is a text file that contains a series of instructions defining how to build a Docker image, including the base image to use, necessary packages to install, and any files to include. By executing the Dockerfile with the docker build command, Docker reads the instructions and constructs the image accordingly, encapsulating the application and its dependencies into a single, portable unit.

Dockerfiles enable automation in the image creation process, ensuring that the image is built consistently and reliably across different environments. This standardized approach streamlines the development and deployment process, making it easier for developers to share their applications through Docker containers. The other options listed either do not exist within the Docker ecosystem or do not specifically serve the purpose of defining Docker images in the same manner.