Move all files on the host under a common directory

This will make running kamal remove simpler, we can just clean up that
directory.

lib/kamal/configuration.rb

@@ -194,6 +194,19 @@ class Kamal::Configuration
     ".kamal"
   end
 
+  def service_directory
+    File.join run_directory, "apps", [ service, destination ].compact.join("-")
+  end
+
+  def env_directory
+    File.join service_directory, "env"
+  end
+
+  def assets_directory
+    File.join service_directory, "assets"
+  end
+
+
   def run_directory_as_docker_volume
     File.join "$(pwd)", run_directory
   end
@@ -207,10 +220,6 @@ class Kamal::Configuration
   end
 
 
-  def env_directory
-    File.join(run_directory, "env")
-  end
-
   def env_tags
     @env_tags ||= if (tags = raw_config.env["tags"])
       tags.collect { |name, config| Env::Tag.new(name, config: config, secrets: secrets) }