Wait for container initialization before devcontainer setup

I'm using a container that has its own init system. Part of the init process of the container is to create the container user. I'm running into an issue where the devcontainer implementation [starts trying to get information about the remoteUser](https://github.com/devcontainers/cli/blob/1da368b5a227acea4b427fea2da4548960a7a061/src/spec-node/singleContainer.ts#L65) before that user is even created.

Is there any method to block first-time container inspection/setup until a command finishes? I was hoping to use `onCreateCommand` for this, but it is run after the container is probed, not immediately after it is created.

#299 might solve this issue.