1.9.0: Bash Syntax used in POSIX Shell-Script

Summary

After Installing GitLab CI Multi Runner 1.9.0 from Ubuntu Repository the following is shown:

Get:1 https://packages.gitlab.com/runner/gitlab-ci-multi-runner/ubuntu xenial/main amd64 gitlab-ci-multi-runner amd64 1.9.0 [34.2 MB]
Fetched 34.2 MB in 3s (10.6 MB/s)
(Reading database ... 193142 files and directories currently installed.)
Preparing to unpack .../gitlab-ci-multi-runner_1.9.0_amd64.deb ...
Unpacking gitlab-ci-multi-runner (1.9.0) over (1.8.1) ...
Setting up gitlab-ci-multi-runner (1.9.0) ...
GitLab Runner: detected user gitlab-runner
Clearing docker cache...
/usr/share/gitlab-runner/clear-docker-cache: 8: /usr/share/gitlab-runner/clear-docker-cache: Syntax error: "(" unexpected

Steps to reproduce

Install Runner Version 1.8.x, do an Upgrade to 1.9.0

Actual behavior

/usr/share/gitlab-runner/clear-docker-cache is not executed due to an syntax error

Expected behavior

Script should run properly.

Relevant logs and/or screenshots

see above

Environment description

root@curiosity ~ # lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.1 LTS
Release:	16.04
Codename:	xenial
root@curiosity ~ #

Used GitLab Runner version

happens during update from 1.8.x to 1.9.0, can be fixed by updatin the shebang from #!/bin/sh to #!/bin/bash (presumed the OS provides a bash)