Add repositoryConfigurationPath variable

[rubensa]

When setting a repository configuration using:

remote.containers.repositoryConfigurationPaths

in the devconstainer.json file I can't reference any relative file to devcontainer.json (for example for initializeCommand or mounts).

It should be very helpful to automatically set a repositoryConfigurationPath variable (like localWorkspaceFolder, for example) and allow it to be replaced inside devcontainer.json.