DevOps is work culture that combines the development and operations team for continuous development, continuous testing, continuous delivery, continuous integration, and continuous monitoring.
On the other hand, Docker is nothing but another tool used to implement this work culture.
Docker encapsulates code and code dependencies in a single unit also known as container that can be run anywhere Docker engine is installed. In terms of Continuous Integration /Continuous Deployment, it can help DevOps Engineers separate Configuration from Code, decrease the amount of time spent doing dependency management etc., can use it to scale (with the help of some other tools of course). The list goes on.