From the source
This question could be dismissed by saying that Nix and Docker are different tools that solve different problems.
One is a toolkit for building and deploying containers and the other is a package and configuration manager.
However, these tools do have some overlap: they can both be used to create reproducible environments.
A reproducible environment is one that can be recreated from scratch in an identical way (ideally bit-for-bit).
Practically, this means having the same tools, versions, and configuration between the environments.
Reproducible environments are useful to ensure all developers on a project have the exact same set of tools.
Additionally, you can develop in an environment that is similar to the production environment -- leading to less surprises when deploying.
Both tools can solve the age-old problem of "it works on my machine".
While both tools aim to solve this problem, they take different approaches.
Reproducible Environment with Docker Docker provides tools for creating container images.
An image stores the contents and configuration for a container.
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