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