Enable privileged mode on Shared Runners

This issue is to track enabling of --privileged mode on Shared Runners:

  • Report security vulnerability in docker-machine to security@docker.com,
  • Create shared-runners-manager-2.gitlab.com,
  • Make shared-runners-manager-1.gitlab.com be managed by Chef,
  • Make shared-runners-manager-2.gitlab.com be managed by Chef,
  • Receive an acknowledge of the docker-machine security issue,
  • Verify a fix provided by Docker,
  • Update shared-runners-manager-1.gitlab.com to GitLab Runner 1.3,
  • Update shared-runners-manager-2.gitlab.com to GitLab Runner 1.3,
  • Update docker-machine on shared-runners-manager-1.gitlab.com,
  • Enable on privileged flag on shared-runners-manager-1.gitlab.com,
  • Update docker-machine on shared-runners-manager-2.gitlab.com,
  • Enable on privileged flag on shared-runners-manager-2.gitlab.com,
  • Document configuration of shared-runners,
  • Prepare blog post with announcement of updated shared runners,

cc @tmaczukin