Version of docker-ce should be pinned to prevent incompatible upgrades

[eliasp]

apt list --upgradable on a newly created node shows:

docker-ce/xenial 17.12.0~ce-0~ubuntu amd64 [upgradable from: 17.03.2~ce-0~ubuntu-xenial]

This would upgrade docker-ce to 17.12.* which is not compatible with current K8S releases.
This can be prevented by creating /etc/apt/preferences.d/docker-ce:

Package: docker-ce
Pin: version 17.03.*
Pin-Priority: 1000

References regarding supported versions:

• K8S 1.7
• K8S 1.8
• K8S 1.9

[eliasp]

Once the version is pinned, the apt-cache/grep workaround won't be needed anymore and a simple apt install -y docker-ce will install the correct docker-ce release.