Nix is a language, package manager, and OS. This post discusses NixOS.
While docker-compose allows you to compose your containers with a yaml/Dockerfiles, NixOS allows you to compose the system that all of your containers run on (from userspace down to kernel selection/configs, file system, etc), as well as your containers - all in a declarative .nix file. That .nix file can be used to spin up any number of identitically configured systems.
It's also reproducible, in that you can specify the sources (refined to a specific commit if you prefer) for any and all packages on the system - and build them with Nix within a sandboxed environment protecting dependencies and env configurations (Nix is also a powerful build system).
While docker-compose allows you to compose your containers with a yaml/Dockerfiles, NixOS allows you to compose the system that all of your containers run on (from userspace down to kernel selection/configs, file system, etc), as well as your containers - all in a declarative .nix file. That .nix file can be used to spin up any number of identitically configured systems.
It's also reproducible, in that you can specify the sources (refined to a specific commit if you prefer) for any and all packages on the system - and build them with Nix within a sandboxed environment protecting dependencies and env configurations (Nix is also a powerful build system).