Docker and Nix are two different tools that aim to solve the problem of "it works on my machine" in software development, but they take different approaches to achieve this goal. Docker provides a toolkit for creating container images, which can be used to create reproducible environments by distributing identical images to developers or shipping services to production. However, Docker's image building process is not always reproducible and has limitations, such as the inability to combine multiple images. Nix, on the other hand, takes a first-principles approach to reproducible builds and package management, providing a whole build system that allows for building packages in an isolated way, with strong guarantees about reproducibility. This makes it easier to share environments and ensure consistency across different machines. While Docker provides rich tooling for containerized deployments, Nix is primarily designed for building packages and environments in a reproducible way, but can be used to build Docker images as well. Ultimately, the choice between Docker and Nix depends on the specific needs of the project, with Nix being a better fit for projects that require reproducible builds and package management.