Move ci step code to ci-templates project

What does this MR do?

This MR moves the code from the check-docker-image-size into a single main.go file in this repo, as discussed here.

What are the relevant issue numbers?

Update check image size job in ci-templates to ... (gitlab-org/gitlab#489488 - closed) • Cameron Swords

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug

  See this comment for tests.