Introduce hadolint

• https://github.com/hadolint/hadolint.git

https://github.com/hadolint/hadolint/blob/master/docs/INTEGRATION.md

lint_dockerfile:
  stage: lint
  image: hadolint/hadolint:latest-debian
  script:
    - hadolint Dockerfile

mentioned in merge request !3 (merged)

closed via commit 1847ce3e

closed via merge request !3 (merged)

mentioned in commit e404d80d

mentioned in commit c47ec0b2