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

lib/kamal/cli/build.rb

@@ -10,6 +10,7 @@ def deliver
   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 @@ def push
         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 @@ def clean
     docker :image, :rm, "--force", config.absolute_image
   end
 
-  def push
+  def push(export_action)
     docker :buildx, :build,
-      "--push",
+      "--output=type=#{export_action}",
       *platform_options(arches),
       *([ "--builder", builder_name ] unless docker_driver?),
       *build_options,

test/cli/build_test.rb

@@ -26,7 +26,30 @@ class CliBuildTest < CliTestCase
         assert_match /Cloning repo into build directory/, output
         assert_match /git -C #{Dir.tmpdir}\/kamal-clones\/app-#{pwd_sha} clone #{Dir.pwd}/, output
         assert_match /docker --version && docker buildx version/, output
-        assert_match /docker buildx build --push --platform linux\/amd64 --builder kamal-local-docker-container -t dhh\/app:999 -t dhh\/app:latest --label service="app" --file Dockerfile \. as .*@localhost/, output
+        assert_match /docker buildx build --output=type=registry --platform linux\/amd64 --builder kamal-local-docker-container -t dhh\/app:999 -t dhh\/app:latest --label service="app" --file Dockerfile \. as .*@localhost/, output
+      end
+    end
+  end
+
+  test "push --output=docker" do
+    with_build_directory do |build_directory|
+      Kamal::Commands::Hook.any_instance.stubs(:hook_exists?).returns(true)
+      hook_variables = { version: 999, service_version: "app@999", hosts: "1.1.1.1,1.1.1.2,1.1.1.3,1.1.1.4", command: "build", subcommand: "push" }
+
+      SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+        .with(:git, "-C", anything, :"rev-parse", :HEAD)
+        .returns(Kamal::Git.revision)
+
+      SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+        .with(:git, "-C", anything, :status, "--porcelain")
+        .returns("")
+
+      run_command("push", "--output=docker", "--verbose").tap do |output|
+        assert_hook_ran "pre-build", output, **hook_variables
+        assert_match /Cloning repo into build directory/, output
+        assert_match /git -C #{Dir.tmpdir}\/kamal-clones\/app-#{pwd_sha} clone #{Dir.pwd}/, output
+        assert_match /docker --version && docker buildx version/, output
+        assert_match /docker buildx build --output=type=docker --platform linux\/amd64 --builder kamal-local-docker-container -t dhh\/app:999 -t dhh\/app:latest --label service="app" --file Dockerfile \. as .*@localhost/, output
       end
     end
   end
@@ -49,7 +72,7 @@ class CliBuildTest < CliTestCase
       SSHKit::Backend::Abstract.any_instance.expects(:execute).with(:git, "-C", build_directory, :submodule, :update, "--init")
 
       SSHKit::Backend::Abstract.any_instance.expects(:execute)
-        .with(:docker, :buildx, :build, "--push", "--platform", "linux/amd64", "--builder", "kamal-local-docker-container", "-t", "dhh/app:999", "-t", "dhh/app:latest", "--label", "service=\"app\"", "--file", "Dockerfile", ".")
+        .with(:docker, :buildx, :build, "--output=type=registry", "--platform", "linux/amd64", "--builder", "kamal-local-docker-container", "-t", "dhh/app:999", "-t", "dhh/app:latest", "--label", "service=\"app\"", "--file", "Dockerfile", ".")
 
       SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
         .with(:git, "-C", anything, :"rev-parse", :HEAD)
@@ -74,7 +97,7 @@ class CliBuildTest < CliTestCase
       assert_no_match /Cloning repo into build directory/, output
       assert_hook_ran "pre-build", output, **hook_variables
       assert_match /docker --version && docker buildx version/, output
-      assert_match /docker buildx build --push --platform linux\/amd64 --builder kamal-local-docker-container -t dhh\/app:999 -t dhh\/app:latest --label service="app" --file Dockerfile . as .*@localhost/, output
+      assert_match /docker buildx build --output=type=registry --platform linux\/amd64 --builder kamal-local-docker-container -t dhh\/app:999 -t dhh\/app:latest --label service="app" --file Dockerfile . as .*@localhost/, output
     end
   end
 
@@ -140,7 +163,7 @@ class CliBuildTest < CliTestCase
         .returns("")
 
       SSHKit::Backend::Abstract.any_instance.expects(:execute)
-        .with(:docker, :buildx, :build, "--push", "--platform", "linux/amd64", "--builder", "kamal-local-docker-container", "-t", "dhh/app:999", "-t", "dhh/app:latest", "--label", "service=\"app\"", "--file", "Dockerfile", ".")
+        .with(:docker, :buildx, :build, "--output=type=registry", "--platform", "linux/amd64", "--builder", "kamal-local-docker-container", "-t", "dhh/app:999", "-t", "dhh/app:latest", "--label", "service=\"app\"", "--file", "Dockerfile", ".")
 
       run_command("push").tap do |output|
         assert_match /WARN Missing compatible builder, so creating a new one first/, output