Persistent connection closed

Summary

Some of jobs are failing with error: ERROR: Job failed (system failure): persistent connection closed

The project uses gradle to run tests. After BUILD SUCCESSFUL giltab build fails with that error. It happens at random and rebuild helps

Relevant logs and/or screenshots

End of gradle logs:

12:22:14.308 [INFO] com.zaxxer.hikari.pool.HikariPool - HikariPool-2 - Closed.

BUILD SUCCESSFUL

Total time: 16 mins 8.545 secs
ERROR: Job failed (system failure): persistent connection closed

Environment description

We have self hosted Gitlab CE (9.0.4). Runner (9.0.1) is using docker executor

# docker info
Containers: 8
 Running: 1
 Paused: 0
 Stopped: 7
Images: 44
Server Version: 1.12.3
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 383
 Dirperm1 Supported: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: host bridge null overlay
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Security Options:
Kernel Version: 3.16.0-4-amd64
Operating System: Debian GNU/Linux 8 (jessie)
OSType: linux
Architecture: x86_64
CPUs: 8
Total Memory: 23.6 GiB
Name: gitlabci5
ID: ****
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Username: toukjenkins
Registry: https://index.docker.io/v1/
WARNING: No memory limit support
WARNING: No swap limit support
WARNING: No kernel memory limit support
WARNING: No oom kill disable support
WARNING: No cpu cfs quota support
WARNING: No cpu cfs period support
Insecure Registries:
 127.0.0.0/8

Used GitLab Runner version

# gitlab-runner --version
Version:      9.0.1
Git revision: a3da309
Git branch:   9-0-stable
GO version:   go1.7.5
Built:        Wed, 05 Apr 2017 11:15:09 +0000
OS/Arch:      linux/amd64

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