Allow disabling of local builds

To disable local builds set:
```
builder:
   local: false
   remote: ssh://docker@docker-builder
```

test/commands/builder_test.rb

@@ -37,6 +37,14 @@ class CommandsBuilderTest < ActiveSupport::TestCase
       builder.push.join(" ")
   end
 
+  test "remote build if remote is set and local disabled" do
+    builder = new_builder_command(builder: { "arch" => [ "amd64", "arm64" ], "remote" => "ssh://app@127.0.0.1", "cache" => { "type" => "gha" }, "local" => false })
+    assert_equal "remote", builder.name
+    assert_equal \
+      "docker buildx build --push --platform linux/amd64,linux/arm64 --builder kamal-remote-ssh---app-127-0-0-1 -t dhh/app:123 -t dhh/app:latest --cache-to type=gha --cache-from type=gha --label service=\"app\" --file Dockerfile .",
+      builder.push.join(" ")
+  end
+
   test "target remote when remote set and arch is non local" do
     builder = new_builder_command(builder: { "arch" => [ "#{remote_arch}" ], "remote" => "ssh://app@host", "cache" => { "type" => "gha" } })
     assert_equal "remote", builder.name

test/configuration/builder_test.rb

@@ -132,6 +132,23 @@ class ConfigurationBuilderTest < ActiveSupport::TestCase
     assert_equal "default=$SSH_AUTH_SOCK", config.builder.ssh
   end
 
+  test "local disabled but no remote set" do
+    @deploy[:builder]["local"] = false
+
+    assert_raises(Kamal::ConfigurationError) do
+      config.builder
+    end
+  end
+
+  test "local disabled all arches are remote" do
+    @deploy[:builder]["local"] = false
+    @deploy[:builder]["remote"] = "ssh://root@192.168.0.1"
+    @deploy[:builder]["arch"] = [ "amd64", "arm64" ]
+
+    assert_equal [], config.builder.local_arches
+    assert_equal [ "amd64", "arm64" ], config.builder.remote_arches
+  end
+
   private
     def config
       Kamal::Configuration.new(@deploy)