Docker is not directly part of either frontend or backend. It is a tool for managing application containerisation and infrastructure, making it more of a DevOps tool.
However, Docker is often used as part of the infrastructure to deploy both frontend and backend applications. For example, you can use Docker to containerise your frontend application, such as a web application written in React or Angular, and to containerise your backend application written in Node.js, Python, Java or another programming language.
Thus, Docker can be used to deploy both frontend and backend components of your application, making it an important tool for developers developing and deploying web applications.