Support `containerEnv:...` variable syntax when setting `cmake.cmakePath` in devcontainer.json

^{Originally posted by **skycaptain** September 12, 2024}
I'm setting up a development environment using devcontainers for a project that uses a cross-compiling SDK.

Our setup defines a path to the native sysroot installation using an environment variable called `SDK_NATIVE_SYSROOT`. We aim to use this variable to set `cmake.cmakePath`. The documentation states that we can use environment variables in the configuration like this: `${env:VARIABLE_NAME}`.

Here's what I've tried in my configuration file:

```
{
"image": "[redacted]",
"customizations": {
"vscode": {
"extensions": [
"ms-vscode.cpptools-extension-pack"
],
"settings": {
"cmake.cmakePath": "${env:SDK_NATIVE_SYSROOT}/usr/bin/cmake"
}
}
}
}
```

However, this doesn't work. When I run it, I get an error saying it can't find CMake at `/usr/bin/cmake`. It seems like `${env:SDK_NATIVE_SYSROOT}` is being replaced with an empty string.

Interestingly, if I put the same setting in `.vscode/settings.json`, it works fine.

It appears that settings in the devcontainer.json file are evaluated using the `localEnv` before being sent to the devcontainer, while settings in `.vscode/settings.json` are evaluated inside the container. However, the `SDK_NATIVE_SYSROOT` variable only exists in the container, not on my local computer.

This behavior seems counterintuitive, as the settings in `devcontainer.json` are meant for the container, not my local machine. Is there a way to define `cmake.cmakePath` exclusively for the container without using `.vscode/settings.json`? Using the latter would also override settings for users working locally.

According to [this](https://github.com/devcontainers/spec/issues/485) issue we should use `${containerEnv:SDK_NATIVE_SYSROOT}`, however this is not supported by this extension.