Debian package seems to overwrite the service config

Summary

Summarize the bug encountered concisely

Steps to reproduce

gitlab-runner uninstall 
gitlab-runner install --user www-data -d "/var/www/html/example/web"
gitlab-runner stop
gitlab-runner start
ps aux|grep runner

root 30841 2.3 0.5 53652 22088 ? Ssl 16:20 0:00 /usr/bin/gitlab-ci-multi-runner run --working-directory /var/www/html/example/web --config /etc/gitlab-runner/config.toml --service gitlab-runner --syslog --user www-data

apt-get upgrade
# upgrades from gitlab-ci-multi-runner:amd64 (9.0.2, 9.1.0) as shown in the apt/history.log

Actual behavior

The /etc/systemd/system/gitlab-runner.service was overwritten

Expected behavior

The service file shall not be touched

There was no interaction during the upgrade (it did not ask to replace the config file). It just was replaced

Environment description

Ubuntu 16.04.2 LTS

Used GitLab Runner version

9.0.2 -> 9.1.0

gitlab-runner --version Version: 9.1.0 Git revision: 0118d89 Git branch: 9-1-stable GO version: go1.7.5 Built: Fri, 21 Apr 2017 14:32:04 +0000 OS/Arch: linux/amd64