Define image for step

Overview

The job API responds with a an array of steps which we use to build the user script and after_script.

In gitlab#38352 (comment 264198104) & gitlab#39089 (comment 259412690) we are proposing to extend the steps so that we can run scripts generated from .gitlab-ci.yml. The tools needed for these steps to run wouldn't be inside image they define if they are running a Container-based executor. Since we are "hiding" these tools from the user by translating the .gitlab-ci.yml they define into commands we need a way for Rails to define image for that specific step so it controls the environment as well.

Proposal

Extend step to define an image which will be interpreted by each executor and will run that step inside of the specified image.