Add support for blocking 'entrypoints'

As the `entrypoints` are long lived, the `entrypoint` scripts runs in the background. Hence, this is not a blocking script for the container startup.
Can we add a new semantics to have "blocking" entrypoints that the CLI waits for?

Scenario: We have seen flakiness in docker startups for docker-in-docker Feature where docker is not running in the container. See https://github.com/devcontainers/features/issues/634 and https://github.com/devcontainers/features/issues/660.

https://github.com/devcontainers/features/pull/669 adds few retry mechanisms to attempt to fix this issue. `/usr/local/share/docker-init.sh` which starts/retries dockerd is added to the `entrypoint` command. This command runs in the background and is not a blocking script for the container startup. Since it's in the background, `onCreateCommand/postCreateCommand/postStartCommand` could all start executing before docker is fully running. If it takes docker too long, that could introduce flakiness in those lifecycle scripts. Hence, it would be great to have some way of adding a blocking entrypoint to ensure dockerd is ready for the container (including the ^ lifecycle scripts)

// cc @chrmarti