Support using config variables in devcontainer.json

I would like to be able to refer to a custom value specified in settings.json inside the devcontainer.json.

For example, if I have the following as an entry in my user-level settings.json:

"my.data_dir": "X:/"

I would like to be able to have:

"mounts": [
    "src=${localConfig:my.data_dir},dst=/mnt/data,type=bind"
]

It should be possible to use this in build.args or runArgs or anywhere else that variables are currently supported. This would allow each user to put these files in a different path.

Current workaround is to put the value in the environment instead and use ${localEnv:MY_DATA} but this seems less elegant as it pollutes all other apps on the system.

This suggestion is intended to mirror ${localConfig:x} to ${config:x} in tasks.json/launch.json similar to how ${localEnv:x} mirrors ${env:x}.

Start by tracing the existing variable expansion used by devcontainer.json, especially ${localEnv}, and how values from settings.json are read. Review the requested mounts example along with build.args and runArgs; done means a user-level setting can be referenced consistently wherever variables are currently supported.