Containers do not do deployment (or set up software defined networks). docker or kubernetes (or others) do deployment. That's my point.

nix makes it trivial to set up ephemeral environments: make a shell.nix file and run `nix-shell` (or if you just need a thing or two, do e.g. `nix-shell -p ffmpeg` and now you're in a shell with ffmpeg. When you close that shell it's gone). You might use something like `direnv` to automate that.

Nixos makes it easy to define your networking setup through config.

For your last question:

    services.postgres.enable = true;
    services.nginx.enable = true;
    services.keycloak.enable = true;

If you want, you can wrap some or all of those lines in a container, e.g.

    containers.backend = {
        config = { config, pkgs, lib, ... }: {
            services.postgres.enable = true;
            services.keycloak.enable = true;
        };
    };

Though you'd presumably want some additional networking and bind mount config (e.g. putting it into its own network namespace with a bridge, or maybe binding domain sockets that nginx will use plus your data partitions).