Combine multiarch and native/cache builders

Combine the two builders, as they are almost identical. The only
difference was whether the platforms were set.

The native cached builder wasn't using the context it created, so now
we do.

We'll set the driver to `docker-container` - it seems to be the default
but the Docker docs claim it is `docker`.

lib/kamal/commands/builder.rb

@@ -11,22 +11,17 @@ class Kamal::Commands::Builder < Kamal::Commands::Base
   end
 
   def target
-    if config.builder.multiarch?
-      if config.builder.remote?
-        if config.builder.local?
-          multiarch_remote
-        else
-          native_remote
-        end
-      else
-        multiarch
-      end
+    case
+    when !config.builder.multiarch? && !config.builder.cached?
+      native
+    when !config.builder.multiarch? && config.builder.cached?
+      local
+    when config.builder.local? && config.builder.remote?
+      multiarch_remote
+    when config.builder.remote?
+      native_remote
     else
-      if config.builder.cached?
-        native_cached
-      else
-        native
-      end
+      local
     end
   end
 
@@ -34,16 +29,12 @@ class Kamal::Commands::Builder < Kamal::Commands::Base
     @native ||= Kamal::Commands::Builder::Native.new(config)
   end
 
-  def native_cached
-    @native ||= Kamal::Commands::Builder::Native::Cached.new(config)
-  end
-
   def native_remote
     @native ||= Kamal::Commands::Builder::Native::Remote.new(config)
   end
 
-  def multiarch
-    @multiarch ||= Kamal::Commands::Builder::Multiarch.new(config)
+  def local
+    @local ||= Kamal::Commands::Builder::Local.new(config)
   end
 
   def multiarch_remote