Minimise holding the deploy lock

If we get an error we'll only hold the deploy lock if it occurs while
trying to switch the running containers.

We'll also move tagging the latest image from when the image is pulled
to just before the container switch. This ensures that earlier errors
don't leave the hosts with an updated latest tag while still running the
older version.

lib/mrsk/commands/app.rb

@@ -128,6 +128,10 @@ class Mrsk::Commands::App < Mrsk::Commands::Base
     docker :image, :prune, "--all", "--force", *filter_args
   end
 
+  def tag_current_as_latest
+    docker :tag, config.absolute_image, config.latest_image
+  end
+
 
   private
     def container_name(version = nil)

lib/mrsk/commands/builder/base.rb

@@ -7,7 +7,6 @@ class Mrsk::Commands::Builder::Base < Mrsk::Commands::Base
 
   def pull
     docker :pull, config.absolute_image
-    docker :pull, config.latest_image
   end
 
   def build_options