DIB elements for Docker

Change-Id: Id7dfdd0390ffec2e19a190ec5f974d96bc178387

contrib/elements/docker/README.md

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+This element install Docker on Ubuntu/CentOS

contrib/elements/docker/install.d/56-docker

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+#!/bin/bash
+
+set -eu
+
+if [ -e /etc/lsb-release ]; then
+    if [ -e /usr/lib/apt/methods/https ]; then
+        apt-get update
+        apt-get install apt-transport-https
+    fi
+    apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 36A1D7869245C8950F966E92D8576A8BA88D21E9
+    echo "deb https://get.docker.com/ubuntu docker main" > /etc/apt/sources.list.d/docker.list
+    apt-get update
+    apt-get -y install lxc-docker
+else
+    yum -y install docker
+fi