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

test/commands/docker_test.rb

@@ -0,0 +1,26 @@
+require "test_helper"
+
+class CommandsDockerTest < ActiveSupport::TestCase
+  setup do
+    @config = {
+      service: "app", image: "dhh/app", registry: { "username" => "dhh", "password" => "secret" }, servers: [ "1.1.1.1" ]
+    }
+    @docker = Mrsk::Commands::Docker.new(Mrsk::Configuration.new(@config))
+  end
+
+  test "install" do
+    assert_equal "curl -fsSL https://get.docker.com | sh", @docker.install.join(" ")
+  end
+
+  test "installed?" do
+    assert_equal "docker -v", @docker.installed?.join(" ")
+  end
+
+  test "running?" do
+    assert_equal "docker version", @docker.running?.join(" ")
+  end
+
+  test "superuser?" do
+    assert_equal '[ "${EUID:-$(id -u)}" -eq 0 ]', @docker.superuser?.join(" ")
+  end
+end