Allow disabling of local builds

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

lib/kamal/configuration/builder.rb

@@ -28,7 +28,9 @@ class Kamal::Configuration::Builder
   end
 
   def local_arches
-    @local_arches ||= if remote
+    @local_arches ||= if local_disabled?
+      []
+    elsif remote
       arches & [ Kamal::Utils.docker_arch ]
     else
       arches
@@ -48,7 +50,7 @@ class Kamal::Configuration::Builder
   end
 
   def local?
-    arches.empty? || local_arches.any?
+    !local_disabled? && (arches.empty? || local_arches.any?)
   end
 
   def cached?
@@ -79,6 +81,10 @@ class Kamal::Configuration::Builder
     builder_config.fetch("driver", "docker-container")
   end
 
+  def local_disabled?
+    builder_config["local"] == false
+  end
+
   def cache_from
     if cached?
       case builder_config["cache"]["type"]

lib/kamal/configuration/docs/builder.yml

@@ -30,6 +30,11 @@ builder:
   # If you have a remote builder, you can configure it here
   remote: ssh://docker@docker-builder
 
+  # Whether to allow local builds
+  #
+  # Defaults to true
+  local: true
+
   # Builder cache
   #
   # The type must be either 'gha' or 'registry'

lib/kamal/configuration/validator/builder.rb

@@ -7,5 +7,7 @@ class Kamal::Configuration::Validator::Builder < Kamal::Configuration::Validator
     end
 
     error "Builder arch not set" unless config["arch"].present?
+
+    error "Cannot disable local builds, no remote is set" if config["local"] == false && config["remote"].blank?
   end
 end

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)