Update Docker Machine to 0.12.2 and add checksum checking for Docker Machine and dumb-init

What does this MR do?

Updated Docker Machine installed in gitlab/gitlab-docker:ubuntu and gitlab/gitlab-docker:alpine images to version 0.12.2. It also adds sha256sum check for downloaded docker-machine and dumb-init binaries.

Why was this MR needed?

Please read #2640 (closed) for reference.

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

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #2640 (closed)

@nick.thomas Could you review and approve the change?

resolved all discussions

Updated!

@tmaczukin one tiny comment, otherwise LGTM!

assigned to @tmaczukin

resolved all discussions

@nick.thomas I've updated the checksum check a little.

assigned to @nick.thomas

@tmaczukin OK, LGTM

approved this merge request

merged