What is a Docker image?

A Docker image is a read-only template used for creating containers. It serves as a blueprint that includes all the necessary components, such as the application code, libraries, environment variables, and configuration files, which are needed for an application to run in a container. When a Docker image is executed, it creates a container instance that can be started, stopped, and managed independently. This allows developers to package applications with all their dependencies in a consistent environment, making it easy to share and deploy across different systems.

The focus on being read-only is significant because it ensures that the underlying image remains unchanged regardless of the state or modifications made to any running containers created from it. This immutability is key to ensuring that containers are reproducible and reliable. Understanding this concept is fundamental in utilizing Docker effectively for application deployment and management.