Start Runner container with dumb-init as the main process.

Hello, Using docker-machine through this chart in ours environments, we found a lot of zombie process in pod gitlab-runner, taking more and more memory. By using an init system, the zombies are killed.

The solution has been fixed in the official image through following MR, and was overriden with the entrypoint.

gitlab-org/gitlab-runner!180 (merged) gitlab-org/gitlab-runner#1366 (closed)