Add a feature to make gitlab-runner member of docker group

[LongLiveCHIEF]

Migrated from voxpupuli/puppet-gitlab#150, originally opened by @gdubicki

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Pleae make this module optionally do step 4. from https://docs.gitlab.com/ce/ci/docker/using_docker_build.html#use-shell-executor in case you want to use the runner for building Docker images.

You can do it with a hacky-but-working:

exec {'add gitlab-runner to docker group':
    command => '/sbin/usermod -aG docker gitlab-runner',
    unless  => '/bin/grep -q "docker\\S*gitlab-runner" /etc/group',
}

[logicminds]

I recently setup a new runner with docker and the step you provided above was not needed and all the jobs ran successfully. I suspect this is the case because when the gitlab-runner service is running it runs as root which has docker privileges. However, the scripts themselves are run as a different user defined like: gitlab-runner run --user gitlab-runner. So I am not sure this is really needed.

gitlab-runner run --help
Runtime platform                                    arch=amd64 os=darwin pid=7005 revision=ac8e767a version=12.6.0
NAME:
   gitlab-runner run - run multi runner service

USAGE:
   gitlab-runner run [command options] [arguments...]

OPTIONS:
   -c value, --config value             Config file (default: "/Users/cosman/.gitlab-runner/config.toml") [$CONFIG_FILE]
   --listen-address value               Metrics / pprof server listening address [$LISTEN_ADDRESS]
   -n value, --service value            Use different names for different services (default: "gitlab-runner")
   -d value, --working-directory value  Specify custom working directory
   -u value, --user value               Use specific user to execute shell scripts
   --syslog                             Log to system service logger [$LOG_SYSLOG]