Docker is kinda the opposite of Nix in this respect— Docker is fundamentally parasitic on other tools for dependency management, and Nix handles dependencies itself.

That parasitism is also Docker's strength: bring along whatever knowledge you have of your favorite language ecosystem's toolchain; it'll not only apply but it'll likely be largely sufficient.

Build systems like Buck and Bazel are more like Nix in this respect: they take over the responsibilities of soke tools in your language's toolchain (usually high-level build tools, sometimes also dependency managers) so they can impose a certain discipline and yield certain benefits (crucially fine-grained, incremental compilation).

Anyway, Docker doesn't fetch or resolve the dependencies of Python packages. It leaves that to other tools (Nix, apt-get, whatever) and just does you the favor of freezing the result as a binary artifact. Immensely useful, but solves a different problem than the main one here, even if it eases some of the same burdens.

Docker is the cached output of a build that just so happened to succeed

Nix guarantees builds succeed so it doesn't need to cache the output