docs: windows: add a note about ContainerUser limited permissions

`ContainerUser` is not permitted to write to `C:\` because of
their limited permissions, `(WD)` at best.

Add a note explaining why we are using `USER ContainerAdministrator`
in our dockerfile for us to be able to run:
```
RUN echo "Goodbye!" >> hello.txt
```

On client SKUs like Win11, there will be no issues since
`ContainerUser` has `(M)` permissions on `C:\`

Fixes moby#4731 as by-design.

Signed-off-by: Anthony Nandaa <profnandaa@gmail.com>

docs/windows.md

@@ -154,6 +154,10 @@ Now that everything is setup, let's build a [simple _hello world_ image](https:/
     This message shows that your installation appears to be working correctly.
     "@
     ```
+
+    > **NOTE:** Writing to a file directly under `C:\` needs extra permissions, hence use of `ContainerAdministrator`.
+    > `ContainerUser` is the default user for `nanoserver` images. See more details at [#4731](https://github.com/moby/buildkit/issues/4731).
+
 1. Build and push to your registry (or set to `push=false`). For Docker Hub, make sure you've done `docker login`. See more details on registry configuration [here](../README.md#imageregistry)
 
     ```powershell