Get rid of dind for Container Scanning

The current job definition is only using dind because we have 2 linked containers: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/lib/gitlab/ci/templates/Security/Container-Scanning.gitlab-ci.yml

I don't see any reason why we can't use a gitlab-runner service:

    - docker run -d --name db arminc/clair-db:latest
    - docker run -p 6060:6060 --link db:postgres -d --name clair --restart on-failure arminc/clair-local-scan:v2.0.6

The rest of the script can (and should) be in a Dockerfile to create a container_scanning image.

Update: unable to use arminc/clair-db:latest and arminc/clair-local-scan:v2.0.6 as services because current version of GitLab Runner does not link service containers to each other, details below.

see &971 (closed) for previous comments