feat: Introduce a build push --output option

which controls where the build result is exported.

The default value is "registry" to reflect the current behavior of
`build push`.

Any value provided to this option will be passed to the `buildx build`
command as a `--output=type=<VALUE>` flag.

For example, the following command will push to the local docker image
store:

    kamal build push --output=docker

squash

lib/kamal/cli/build.rb

@@ -5,11 +5,12 @@ class Kamal::Cli::Build < Kamal::Cli::Base
 
   desc "deliver", "Build app and push app image to registry then pull image on servers"
   def deliver
-    push
-    pull
+    invoke :push
+    invoke :pull
   end
 
   desc "push", "Build and push app image to registry"
+  option :output, type: :string, default: "registry", banner: "export_type", desc: "Exported type for the build result, and may be any exported type supported by 'buildx --output'."
   def push
     cli = self
 
@@ -49,7 +50,7 @@ class Kamal::Cli::Build < Kamal::Cli::Base
         end
 
         # Get the command here to ensure the Dir.chdir doesn't interfere with it
-        push = KAMAL.builder.push
+        push = KAMAL.builder.push(cli.options[:output])
 
         KAMAL.with_verbosity(:debug) do
           Dir.chdir(KAMAL.config.builder.build_directory) { execute *push }

lib/kamal/commands/builder/base.rb

@@ -13,9 +13,9 @@ class Kamal::Commands::Builder::Base < Kamal::Commands::Base
     docker :image, :rm, "--force", config.absolute_image
   end
 
-  def push
+  def push(export_action = "registry")
     docker :buildx, :build,
-      "--push",
+      "--output=type=#{export_action}",
       *platform_options(arches),
       *([ "--builder", builder_name ] unless docker_driver?),
       *build_options,