Bootstrap: use multi-platform installer

* Limit auto-install to root users; otherwise, give manual install guidance
* Support non-Debian/Ubuntu with the multi-OS get.docker.com installer

lib/mrsk/commands/docker.rb

@@ -0,0 +1,21 @@
+class Mrsk::Commands::Docker < Mrsk::Commands::Base
+  # Install Docker using the https://github.com/docker/docker-install convenience script.
+  def install
+    pipe [ :curl, "-fsSL", "https://get.docker.com" ], :sh
+  end
+
+  # Checks the Docker client version. Fails if Docker is not installed.
+  def installed?
+    docker "-v"
+  end
+
+  # Checks the Docker server version. Fails if Docker is not running.
+  def running?
+    docker :version
+  end
+
+  # Do we have superuser access to install Docker and start system services?
+  def superuser?
+    [ '[ "${EUID:-$(id -u)}" -eq 0 ]' ]
+  end
+end