[doc] update nix usage for building docker image

README.md

@@ -72,6 +72,12 @@ docker pull ghcr.io/chipsalliance/t1-$config:latest
 docker pull ghcr.io/chipsalliance/t1-blastoise:latest
 ```
 
+Or build the image using nix and load it into docker
+```bash
+nix build -L ".#t1.$config.release.docker-layers.final-image"
+docker load -i ./result/image.tar
+```
+
 ### Nix setup
 We use Nix Flake as our primary build system. If you have not installed nix, install it following the [guide](https://nixos.org/manual/nix/stable/installation/installing-binary.html), and enable flake following the [wiki](https://nixos.wiki/wiki/Flakes#Enable_flakes). Or you can try the [installer](https://github.com/DeterminateSystems/nix-installer) provided by Determinate Systems, which enables flake by default.