gitlab/gitlab-runner docker image should be ARM compatible

Description

As far as I saw in the code the gitlab-runner binaries itself are ARM compatible. The Docker images though (https://hub.docker.com/r/gitlab/gitlab-runner/) are based on ubuntu (or alpine) and only x84_64 compatible.

Proposal

To be able to run the gitlab-runner container on Kubernetes (or more generally Docker) in an ARM environment the Docker images should be built additionally as ARM compatible versions.

Links to related issues and merge requests / references

The issue came up while I was trying to run Kubernetes on a Raspberry Pi inspired by this blog post.