Why my multiprocess PHP CLI command into docker work as single process?

So, I made cli script using async lib. As I understood right that lib creates child process for every task and executes them parallel. It works perfectly on my local environment.
But when I try to use it via docker container looks like my script work as a single-process (too slow) and from child processes, nothing gets into stdout.
I am not an expert in docker and right now I really don’t know where I need to look for.

Thanks for any advice.

Source: StackOverflow