Add a require_destination setting

If you always want to use a destination, and have a base deploy.yml file
that doesn't specify any hosts, then if you forget to specific the
destination you will get a cryptic error.

Add a "require_destination" setting you can use to avoid this.

lib/kamal/configuration.rb

@@ -9,8 +9,7 @@ class Kamal::Configuration
   delegate :service, :image, :servers, :env, :labels, :registry, :stop_wait_time, :hooks_path, to: :raw_config, allow_nil: true
   delegate :argumentize, :optionize, to: Kamal::Utils
 
-  attr_accessor :destination
-  attr_accessor :raw_config
+  attr_reader :destination, :raw_config
 
   class << self
     def create_from(config_file:, destination: nil, version: nil)
@@ -120,6 +119,10 @@ class Kamal::Configuration
     "#{service}-#{version}"
   end
 
+  def require_destination?
+    raw_config.require_destination
+  end
+
 
   def volume_args
     if raw_config.volumes.present?
@@ -161,7 +164,7 @@ class Kamal::Configuration
   end
 
   def valid?
-    ensure_required_keys_present && ensure_valid_kamal_version
+    ensure_destination_if_required && ensure_required_keys_present && ensure_valid_kamal_version
   end
 
 
@@ -243,6 +246,14 @@ class Kamal::Configuration
       true
     end
 
+    def ensure_destination_if_required
+      if require_destination? && destination.nil?
+        raise ArgumentError, "You must specify a destination"
+      end
+
+      true
+    end
+
 
     def role_names
       raw_config.servers.is_a?(Array) ? [ "web" ] : raw_config.servers.keys.sort

test/configuration_test.rb

@@ -210,6 +210,18 @@ class ConfigurationTest < ActiveSupport::TestCase
     end
   end
 
+  test "destination required" do
+    dest_config_file = Pathname.new(File.expand_path("fixtures/deploy_for_required_dest.yml", __dir__))
+
+    assert_raises(ArgumentError) do
+      config = Kamal::Configuration.create_from config_file: dest_config_file
+    end
+
+    assert_nothing_raised do
+      config = Kamal::Configuration.create_from config_file: dest_config_file, destination: "world"
+    end
+  end
+
   test "to_h" do
     expected_config = \
       { :roles=>["web"],

test/fixtures/deploy_for_required_dest.world.yml

@@ -0,0 +1,5 @@
+servers:
+  - 1.1.1.1
+  - 1.1.1.2
+env:
+  REDIS_URL: redis://x/y

test/fixtures/deploy_for_required_dest.yml

@@ -0,0 +1,7 @@
+service: app
+image: dhh/app
+registry:
+  server: registry.digitalocean.com
+  username: <%= "my-user" %>
+  password: <%= "my-password" %>
+require_destination: true