Process zombie problem in container

Run the docker image.
When you raise a container, you run one daemon server.
The daemon server does not fork and only a few threads are created
If you stop the daemon server in bash, it goes through the normal shutdown phase and runs to exit (0).
But since then, the process becomes zombie.
I wonder why this is the problem. I would also like to find a solution.

Client:
Version: 18.04.0-ce
API version: 1.37
Go version: go1.9.4
Git commit: 3d479c0
Built: Tue Apr 10 18:20:32 2018
OS/Arch: linux/amd64
Experimental: false
Orchestrator: swarm

Server:
Engine:
Version: 18.04.0-ce
API version: 1.37 (minimum version 1.12)
Go version: go1.9.4
Git commit: 3d479c0
Built: Tue Apr 10 18:18:40 2018
OS/Arch: linux/amd64
Experimental: false