You’re comparing apples to oranges. One is a declarative Linux system environment creation solution and the other a daemon that starts sub-system environments using Linux namespaces.
You could in theory use NixOS to define a system environment that you’d run inside of a docker container. It’s a bit harder to get systemd running inside of Docker which NixOS heavily relies on but that’s beside the point. Easier integrations exist for LXD and systemd-nspawn which actually fulfil an equivalent purpose to Docker. The single component that is most comparable to Docker in a typical NixOS deployment would arguably be its init process (systemd), though its use extends far beyond setting up the namespace (the root namespace in this case).
You’re comparing apples to oranges. One is a declarative Linux system environment creation solution and the other a daemon that starts sub-system environments using Linux namespaces.
You could in theory use NixOS to define a system environment that you’d run inside of a docker container. It’s a bit harder to get systemd running inside of Docker which NixOS heavily relies on but that’s beside the point. Easier integrations exist for LXD and systemd-nspawn which actually fulfil an equivalent purpose to Docker. The single component that is most comparable to Docker in a typical NixOS deployment would arguably be its init process (systemd), though its use extends far beyond setting up the namespace (the root namespace in this case).