to preface, I'm not saying that you don't know this, but maybe for others:
gentoo is not a linux distro either (from a certain point of view), more a meta-distribution, where you use the package manager (portage) to build everything from source.
fun fact: docker uses alpine linux, which for a while was a gentoo build, until it got to a point where it could bootstrap itself.
> Nix is like a kind of Docker from an alternative universe.
Nix is often compared to Docker, but Docker is essentially just a way to run programs (in a sandbox). Saying "I'll build it with Docker" says how you're going to run it, but not what you're going to run. Unfortunately, the "what" usually turns out to be a shell script, which runs one package manager to fetch another package manager, downloads a bunch of files from some random URLs, etc.
Nix is all about keeping careful track of the dependencies between things, such that all the fetching has been done by the time our scripts start (access to the network, and any paths outside the build sandbox, is disabled by default).
Also, the way Docker tends to be used involves downloading an entire Linux distro, just to run those scripts. Nix does the opposite: scripts only have access to precisely what they've asked for.
gentoo is not a linux distro either (from a certain point of view), more a meta-distribution, where you use the package manager (portage) to build everything from source.
fun fact: docker uses alpine linux, which for a while was a gentoo build, until it got to a point where it could bootstrap itself.