In Docker, what are labels used for?

Labels in Docker serve as a mechanism for managing and categorizing Docker objects, such as containers, images, networks, and volumes. They consist of key-value pairs that allow users to add metadata to these objects. This functionality is particularly useful for organizing and filtering items based on specific criteria, which can enhance the overall management and orchestration of containers in complex environments.

For instance, by utilizing labels, a user can easily group containers by their function, environment (development, testing, production), or any other custom categorization that aids in identifying and managing a Docker setup. When deploying in larger, multi-container applications, labels can also facilitate the use of tools or command-line interfaces that support filtering or querying based on these labels.

While other options mention functionalities that can be important in the Docker environment—like backups, image size reduction, and performance tracking—they do not accurately reflect the primary purpose of labels within Docker. Labels are not involved in creating backups, directly influencing image sizes, or monitoring performance metrics. Instead, their core utility is in categorization and management, making option B the correct choice.