automatically install docker

[robinbraemer]

It would improve the user experience if RKE installs docker on the nodes while following the suggested installation such as creating a non-root docker user.

I think this feature is a must have for every Kubernetes Engine.
It also makes adding nodes and the setup process much faster and easier since one does not have to ssh manually into every node to install docker while looking at how docker is installed for the different operating systems.

[stale]

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