Extract builder from app

Building is different from running

lib/mrsk/commands/builder.rb

@@ -0,0 +1,15 @@
+require "mrsk/commands/base"
+
+class Mrsk::Commands::Builder < Mrsk::Commands::Base
+  def create
+    docker :buildx, :create, "--use", "--name", "mrsk"
+  end
+
+  def push
+    docker :buildx, :build, "--push", "--platform linux/amd64,linux/arm64", "-t", config.absolute_image, "."
+  end
+
+  def pull
+    docker :pull, config.absolute_image
+  end
+end