What is the primary purpose of a Docker container?

The primary purpose of a Docker container is to run applications in isolated environments. This is one of the cornerstone features that Docker offers, enabling developers to package their applications and all of their dependencies into a lightweight, portable container.

By encapsulating the application along with its environment, Docker containers ensure that the application runs consistently across different computing environments, whether it's on a developer's local machine, a testing server, or in a production environment in the cloud. This isolation allows multiple containers to run on the same host without interfering with each other, providing an efficient way to utilize system resources while maintaining application performance and security.

In contrast, the other choices focus on different functionalities that are not the primary intent of containers. For example, while storing application source code is critical in software development, it is not the main function of a Docker container. Similarly, data backup and secure management of network traffic could involve containers but are not the primary purpose. The essence of containers revolves around the capability to execute applications with all necessary dependencies in a secure, isolated manner, leading to enhanced portability and consistency.