Docker is not only about dependency management. It also offers service "composing", via docker compose, and network isolation for each service.
Although I personally love Nix, and I run NixOS on some of my servers, I do not believe it can replace Docker/Podman. Unless you go the NixOS Containers route.
I am trying to understand.
Docker, which uses OCI containers that are supported by Docker, Podman, Containerd, systemd-nspawn, etc, is lock-in.
But Nix Shells, which require Nix, are not lock-in.
Also, how are you going to run Nix shells in VLANs? They run on the host's network namespace.