add docker docs

README.md

@@ -81,6 +81,26 @@ These files are for either `nix-shell` (default nix) or `nix develop` (flakes):
 
 - `rust` : This is grabs cargo, rustc and others from nixpkgs but also sets the RUST_BACKTRACE variable as 1 to enable it.
 
+## Docker Nix files: (nix-configs/docker/)
+
+These files are for building docker images with `nix-build` such as:
+
+```bash
+nix-build hello-docker.nix
+```
+
+then load the image:
+
+```
+docker load < result
+```
+
+and enter the container:
+
+```
+docker run -it hello sh
+```
+
 ### Screenshots
 
 ![COSMIC Installation](screenshots/NixOS-COSMIC.png)