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

[skycaptain]

Discussed in #4060

^{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 issue we should use ${containerEnv:SDK_NATIVE_SYSROOT}, however this is not supported by this extension.