Fail fast when `devcontainer.json` uses unsupported variables

Relates to #2888

As described by https://code.visualstudio.com/docs/remote/devcontainerjson-reference#_variables-in-devcontainerjson some variables (currently ${localWorkspaceFolder} and ${localWorkspaceFolderBasename}) are not available in devcontainer.json when cloning a repository (instead of opening a local folder).

It appears the Remote Containers extension does not detect this and passes the variable strings literally to Docker which then results in cryptic errors such as:

[34806 ms] Remote-Containers server terminated (code: 137, signal: null).
docker: Error response from daemon: create ${localWorkspaceFolder}/hex/spec: "${localWorkspaceFolder}/hex/spec" includes invalid characters for a local volume name, only "[a-zA-Z0-9][a-zA-Z0-9_.-]" are allowed. If you intended to pass a host directory, use absolute path.
See 'docker run --help'.

Since one of the main purposes of this extension is to simplify starting development on a new project, having such an error is pretty awful.

Could the extension please detect usage of such unsupported variables and fail fast, with a proper error message, before trying to start docker?