Upgrade Docker, from 17.09 to 17.12

[afbjorklund]

The reason that 17.12.0 didn't work (in #2427) was because DOCKER_RAMDISK stopped working in containerd: moby/moby#35957 But now that 17.12.1 has been released as "stable", it should be OK ?

17.12.1-ce (2018-02-27)
- Honor DOCKER_RAMDISK with containerd 1.0 moby/moby#35957

Alternatively, one could upgrade from 17.09.0 to 17.09.1 if there is an issue - or wanting to play it safe...

Kubernetes still doesn't "support" anything beyond Docker 17.03, as pointed out so nicely by kubeadm:

WARNING: docker version is greater than the most recently validated version.

So let's go with the latest (stable), unless there is a reason to stick with the earlier (stable) Sep version ?

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[r2d4]

@praveenkumar should we be concerned about failing minishift test?

[praveenkumar]

should we be concerned about failing minishift test?

@r2d4 No, I went through the logs and not able to find what causes the failure, minishift start happen successfully and even other stuff worked. If you can retrigger this job that would be great.

[r2d4]

@minikube-bot retest this please