Specify which "user" shall run the job (docker-executor)

Description

By default and unless a Docker image as set a default user, an image is run as root user. So when a user select image: ruby:2.4 or image: maven:3-jdk-8, the user that will execute the ruby program or build the project will run as root.

Currently there is no way to define a default user inside the GitLab Runner's config.toml file nor to override it inside the .gitlab-ci.yml file.

Proposal

1. Add an option to enable a runner administrator the ability to define a default user to use for running the CI job container (aka override the default user).

2. A GitLab CI user can define in its .gitlab-ci.yml job description that the image should be jetty and that the user should be jetty as well.
   e.g. image_user: jetty or something more complex:

image:
  name: jetty:latest
  user: jetty

Links to related issues and merge requests / references

None yet.