Build & release autoscaler docker image

What does this MR do?

Creates an autoscaler docker image based on the main gitlab-runner image.

It is released in the same way as the autoscaler binary:

• MRs build & push autoscaler:mr-IID
• master builds & pushes autoscaler:latest
• stable and RC tags build & push autoscaler:TAG

Why was this MR needed?

Deploying the autoscaler in kubernetes

What's the best way to test this MR?

Pull the image from this MR and checks that it contains the autoscaler binary:

docker run -ti --rm --entrypoint "" registry.gitlab.com/gitlab-org/ci-cd/custom-executor-drivers/autoscaler:mr-132 /usr/local/bin/autoscaler

What are the relevant issue numbers?

#65 (closed)

Closes #7 (closed)