exec command frequently exits before running step

There is probably some timing/signal bug. Because step is run once in three or four tries.

uname -a: Linux devpc 4.2.0-34-generic #39-Ubuntu SMP Thu Mar 10 22:13:01 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
docker info:

Containers: 971
 Running: 1
 Paused: 0
 Stopped: 970
Images: 1715
Server Version: 1.10.3
Storage Driver: overlay
 Backing Filesystem: extfs
Execution Driver: native-0.2
Logging Driver: json-file
Plugins: 
 Volume: local
 Network: bridge null host
Kernel Version: 4.2.0-34-generic
Operating System: Ubuntu 15.10
OSType: linux
Architecture: x86_64
CPUs: 8
Total Memory: 15.59 GiB
Name: loki
ID: REDACTED :-)

Dockerfile (docker build -t ci-runner .)

FROM docker:1.9

# gitlab-ci-runner requirements
RUN apk --update-cache add \
		git \
		bash \
		make

# custom requirements
# ...

# workaround to avoid compilation for alpine linux
RUN wget -O /tmp/glibc.apk "https://circle-artifacts.com/gh/andyshinn/alpine-pkg-glibc/6/artifacts/0/home/ubuntu/alpine-pkg-glibc/packages/x86_64/glibc-2.21-r2.apk" \
	&& apk add --allow-untrusted /tmp/glibc.apk \
	&& /usr/glibc/usr/bin/ldconfig /lib /usr/glibc/usr/lib \
	&& rm /tmp/glibc.apk
COPY gitlab-ci-multi-runner /usr/local/bin/

# copy custom scripts
# ...


VOLUME ["/etc/gitlab-runner"]
ENTRYPOINT ["docker-entrypoint.sh"]
CMD ["gitlab-ci-multi-runner", "run"]

Run in root of git repository with .gitlab-ci.yml file:

docker run -it --rm \
	-v "/var/run/docker.sock:/var/run/docker.sock" \
	-v "$(pwd):/source" \
	-w "/source" \
	--entrypoint "gitlab-ci-multi-runner" \
	ci-runner exec shell --builds-dir /tmp build

I have encountered only these outputs on failure:

 - Received signal child exited
 - Grim reaper cleanup: pid=19, wstatus=0
FATAL: wait: no child processes  

 - Received signal child exited
FATAL: wait: no child processes                    
 - Grim reaper cleanup: pid=19, wstatus=0

 - Received signal child exited
 - Received signal child exited
 - Received signal child exited
 - Grim reaper cleanup: pid=21, wstatus=0
 - Received signal child exited

 - Received signal child exited
 - Received signal child exited
 - Received signal child exited
FATAL: wait: no child processes                    
 - Grim reaper cleanup: pid=21, wstatus=0

 - Received signal child exited
 - Received signal child exited
 - Grim reaper cleanup: pid=20, wstatus=0
 - Received signal child exited
 - Grim reaper cleanup: pid=21, wstatus=0
FATAL: wait: no child processes  

Success:

 - Received signal child exited
 - Received signal child exited
 - Received signal child exited
 - Received signal child exited
... build output as expected ...