Make gitlab-runner:alpine more specific, Add link to Dockerfiles sources,...

What does this MR do?

Make gitlab-runner:alpine more specific, Add link to Dockerfiles sources, Remove non-saying list, update image sizes

for i in alpine latest; do
  docker pull "gitlab/gitlab-runner:${i}"
  docker image inspect "gitlab/gitlab-runner:${i}" --format='{{.Size}}'
done

Why was this MR needed?

Confusing instructions (how to get to the alpine image), outdated details (image size)

Are there points in the code the reviewer needs to double check?

Docker image size if you want to.