Add gitlab-task-runner

A container for running rake arbitrary rake tasks

I thought about this while working on the backup and restore creating a new image with the needed dependencies to run the rake task (to close https://gitlab.com/charts/helm.gitlab.io/issues/309).

This is a more generic image, using a container of this we can run gitlab:backup:create and gitlab:backup:restore as well as subtasks such as gitlab:backup:*:create

We pass the task name through specs.container.args

Will be useful in jobs/cronjobs