Zsh scripts leave defunct processes when running under docker exec

[Soares Chen <soares.chen@gmail.com>]

This only happens when the script is running inside a shell spawned by
`docker exec`. When running under normal system and under `docker
run`, there is no defunct process.

This bug is first discovered when I run nvm in docker
(https://github.com/creationix/nvm/issues/650). It was then discovered
that Docker's nsenter did not properly handle the SIGCHLD signal
raised from zsh. (https://github.com/docker/libcontainer/pull/369)
Even though it is now been fixed by Docker, I am not sure why zsh
would cause this bug in the first place.

I have a test script at the end of this post and also share it on
https://gist.github.com/soareschen/240e49116c7f2632d179

Steps to reproduce:

# terminal 1
docker run -it --rm --name test-zsh ubuntu:latest /bin/bash
apt-get install zsh curl
curl https://gist.githubusercontent.com/soareschen/240e49116c7f2632d179/raw/0be67acefd8d18fd62bb181998996f9a5772dc64/docker-zsh-test.sh
> docker-zsh-test.sh
chmod +x docker-zsh-test.sh
./docker-zsh-test.sh
ps auxf # no defunct process

# terminal 2
docker exec -it test-zsh /bin/zsh
./docker-zsh-test.sh
ps auxf # sed and zsh shown as defunct processes

#!/bin/zsh
# docker-zsh-test.sh
if [[ "$(echo "the black cat was chased by the brown dog" | sed -e
's/cat/fox/g' | grep fox)" = "" ]]; then
  echo "cat"
else
  echo "fox"
fi