Pull latest image tag, so we can identity it

`docker image ls` doesn't tell us what the latest deployed image is (e.g
if we've rolled back). Pull the latest image tag through to the server
so we can use it instead.

lib/mrsk/cli/app.rb

@@ -2,7 +2,7 @@ class Mrsk::Cli::App < Mrsk::Cli::Base
   desc "boot", "Boot app on servers (or reboot app if already running)"
   def boot
     say "Get most recent version available as an image...", :magenta unless options[:version]
-    using_version(options[:version] || most_recent_version_available) do |version|
+    using_version(version_or_latest) do |version|
       say "Start container with version #{version} using a #{MRSK.config.readiness_delay}s readiness delay (or reboot if already running)...", :magenta
 
       cli = self
@@ -70,7 +70,7 @@ class Mrsk::Cli::App < Mrsk::Cli::Base
 
     when options[:interactive]
       say "Get most recent version available as an image...", :magenta unless options[:version]
-      using_version(options[:version] || most_recent_version_available) do |version|
+      using_version(version_or_latest) do |version|
         say "Launching interactive command with version #{version} via SSH from new container on #{MRSK.primary_host}...", :magenta
         run_locally { exec MRSK.app.execute_in_new_container_over_ssh(cmd, host: MRSK.primary_host) }
       end
@@ -88,7 +88,7 @@ class Mrsk::Cli::App < Mrsk::Cli::Base
 
     else
       say "Get most recent version available as an image...", :magenta unless options[:version]
-      using_version(options[:version] || most_recent_version_available) do |version|
+      using_version(version_or_latest) do |version|
         say "Launching command with version #{version} from new container...", :magenta
         on(MRSK.hosts) do |host|
           execute *MRSK.auditor.record("Executed cmd '#{cmd}' on app version #{version}"), verbosity: :debug
@@ -189,20 +189,13 @@ class Mrsk::Cli::App < Mrsk::Cli::Base
       end
     end
 
-    def most_recent_version_available(host: MRSK.primary_host)
-      version = nil
-      on(host) { version = capture_with_info(*MRSK.app.most_recent_version_from_available_images).strip }
-      
-      if version == "<none>"
-        raise "Most recent image available was not tagged with a version (returned <none>)"
-      else
-        version.presence
-      end
-    end
-
     def current_running_version(host: MRSK.primary_host)
       version = nil
       on(host) { version = capture_with_info(*MRSK.app.current_running_version).strip }
       version.presence
     end
+
+    def version_or_latest
+      options[:version] || "latest"
+    end
 end

lib/mrsk/commands/app.rb

@@ -87,19 +87,6 @@ class Mrsk::Commands::App < Mrsk::Commands::Base
       "tail -n 1"
   end
 
-  def most_recent_version_from_available_images
-    pipe \
-      docker(:image, :ls, "--format", '"{{.Tag}}"', config.repository),
-      "head -n 1"
-  end
-
-  def all_versions_from_available_containers
-    pipe \
-      docker(:image, :ls, "--format", '"{{.Tag}}"', config.repository),
-      "head -n 1"
-  end
-
-
   def list_containers
     docker :container, :ls, "--all", *filter_args
   end

lib/mrsk/commands/builder/base.rb

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