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/cli/server.rb

@@ -1,17 +1,21 @@
 class Mrsk::Cli::Server < Mrsk::Cli::Base
-  desc "bootstrap", "Ensure curl and Docker are installed on servers"
+  desc "bootstrap", "Set up Docker to run MRSK apps"
   def bootstrap
-    with_lock do
-      on(MRSK.hosts + MRSK.accessory_hosts) do
-        dependencies_to_install = Array.new.tap do |dependencies|
-          dependencies << "curl" unless execute "which curl", raise_on_non_zero_exit: false
-          dependencies << "docker.io" unless execute "which docker", raise_on_non_zero_exit: false
-        end
+    missing = []
 
-        if dependencies_to_install.any?
-          execute "apt-get update -y && apt-get install #{dependencies_to_install.join(" ")} -y"
+    on(MRSK.hosts | MRSK.accessory_hosts) do |host|
+      unless execute(*MRSK.docker.installed?, raise_on_non_zero_exit: false)
+        if execute(*MRSK.docker.superuser?, raise_on_non_zero_exit: false)
+          info "Missing Docker on #{host}. Installing…"
+          execute *MRSK.docker.install
+        else
+          missing << host
         end
       end
     end
+
+    if missing.any?
+      raise "Docker is not installed on #{missing.join(", ")} and can't be automatically installed without having root access and the `curl` command available. Install Docker manually: https://docs.docker.com/engine/install/"
+    end
   end
 end