Proposal: Allow using env-vars in devcontainer-feature.json

My current use case involves defining additional container mounts in the future users home directory in the container. It seems to be impossible at the moment, since the `remoteUser` is configurable.

`${devcontainerId}` seems to be the only variable that can be used in the `devcontainer-feature.json` file. There are 2 env variables mentioned in the feature spec (`_REMOTE_USER_HOME` and `_CONTAINER_USER_HOME`) but they are only accessible in the `install.sh`-script.

I'd like to have access to more variables, either the ones passed to the install-script, or the ones used in the parents `devcontainer.json`.