Connect to remote host before creating builder

Connecting to the remote host will make any SSH configuration issues
obvious and add the host to known hosts if that is how SSHKit is
configured.

lib/kamal/cli/build.rb

@@ -1,3 +1,5 @@
+require "uri"
+
 class Kamal::Cli::Build < Kamal::Cli::Base
   class BuildError < StandardError; end
 
@@ -55,6 +57,10 @@ class Kamal::Cli::Build < Kamal::Cli::Base
   desc "create", "Create a build setup"
   def create
     mutating do
+      if (remote_host = KAMAL.config.builder.remote_host)
+        connect_to_remote_host(remote_host)
+      end
+
       run_locally do
         begin
           debug "Using builder: #{KAMAL.builder.name}"
@@ -103,4 +109,12 @@ class Kamal::Cli::Build < Kamal::Cli::Base
         end
       end
     end
+
+    def connect_to_remote_host(remote_host)
+      remote_uri = URI.parse(remote_host)
+      options = { user: remote_uri.user, port: remote_uri.port }.compact
+      on(remote_uri.host, options) do
+        execute "true"
+      end
+    end
 end

test/cli/build_test.rb

@@ -66,6 +66,14 @@ class CliBuildTest < CliTestCase
     end
   end
 
+  test "create remote" do
+    run_command("create", fixture: :with_remote_builder).tap do |output|
+      assert_match "Running /usr/bin/env true on 1.1.1.5", output
+      assert_match "docker context create kamal-app-native-remote-amd64 --description 'kamal-app-native-remote amd64 native host' --docker 'host=ssh://app@1.1.1.5'", output
+      assert_match "docker buildx create --name kamal-app-native-remote kamal-app-native-remote-amd64 --platform linux/amd64", output
+    end
+  end
+
   test "create with error" do
     stub_setup
     SSHKit::Backend::Abstract.any_instance.stubs(:execute)
@@ -95,8 +103,8 @@ class CliBuildTest < CliTestCase
   end
 
   private
-    def run_command(*command)
+    def run_command(*command, fixture: :with_accessories)
-      stdouted { Kamal::Cli::Build.start([*command, "-c", "test/fixtures/deploy_with_accessories.yml"]) }
+      stdouted { Kamal::Cli::Build.start([*command, "-c", "test/fixtures/deploy_#{fixture}.yml"]) }
     end
 
     def stub_dependency_checks

test/fixtures/deploy_with_remote_builder.yml

@@ -0,0 +1,41 @@
+service: app
+image: dhh/app
+servers:
+  web:
+    - "1.1.1.1"
+    - "1.1.1.2"
+  workers:
+    - "1.1.1.3"
+    - "1.1.1.4"
+registry:
+  username: user
+  password: pw
+
+accessories:
+  mysql:
+    image: mysql:5.7
+    host: 1.1.1.3
+    port: 3306
+    env:
+      clear:
+        MYSQL_ROOT_HOST: '%'
+      secret:
+        - MYSQL_ROOT_PASSWORD
+    files:
+      - test/fixtures/files/my.cnf:/etc/mysql/my.cnf
+    directories:
+      - data:/var/lib/mysql
+  redis:
+    image: redis:latest
+    roles:
+      - web
+    port: 6379
+    directories:
+      - data:/data
+
+readiness_delay: 0
+
+builder:
+  remote:
+    arch: amd64
+    host: ssh://app@1.1.1.5