Tag image after booting and include destination

If you are deploying more than one destination to a host, the latest
tags will conflict, so we'll append the destination to the tag.

The latest tag is used when booting the app or exec-ing a new container.

If a deploy doesn't complete on a host for all roles then we should
probably not be using it, so move the tagging to the end of the boot
process.

lib/kamal/configuration.rb

@@ -128,7 +128,7 @@ class Kamal::Configuration
   end
 
   def latest_image
-    "#{repository}:latest"
+    "#{repository}:#{[ "latest", *destination ].join("-")}"
   end
 
   def service_with_version