Request: add variable for remoteUser

I'd like to be able to access the `remoteUser` field as a devcontainer variable in the devcontainer.json file. For example, when defining cache volumes, I currently have to hardcode my remoteUser's name into the target path:

```json
"mounts": [
{
"source": "brew",
"target": "/home/dev/.cache/Homebrew",
"type": "volume"
},
{
"source": "conan",
"target": "/home/dev/.conan2",
"type": "volume"
},
{
"source": "uv",
"target": "/home/dev/.cache/uv",
"type": "volume"
}
],
```