Allow ssh/config to additionally accept a string

This is accepted by Net::SSH, research done by @jeremy in #908 (comment)

This is already documented as working correctly in https://github.com/basecamp/kamal/blob/74a06b0ccda616c86ebe1729d0795f39bcac9f00/lib/kamal/configuration/docs/ssh.yml#L65-L70

However, before this change only booleans were allowed because of the example configuration file.

lib/kamal/commands/base.rb

@@ -101,6 +101,8 @@ def ssh_config_args
           ""
         when false
           " -F none"
+        when String
+          " -F #{Shellwords.escape(config.ssh.config)}"
         end
       end
   end

lib/kamal/configuration/ssh.rb

@@ -7,7 +7,7 @@ class Kamal::Configuration::Ssh
 
   def initialize(config:)
     @ssh_config = config.raw_config.ssh || {}
-    validate! ssh_config
+    validate! ssh_config, with: Kamal::Configuration::Validator::Ssh
   end
 
   def user

lib/kamal/configuration/validator/ssh.rb

@@ -0,0 +1,19 @@
+class Kamal::Configuration::Validator::Ssh < Kamal::Configuration::Validator
+  SPECIAL_KEYS = [ "config" ]
+
+  def validate!
+    validate_against_example! \
+      config.except(*SPECIAL_KEYS),
+      example.except(*SPECIAL_KEYS)
+
+    validate_config_key! if config.key?("config")
+  end
+
+  private
+
+  def validate_config_key!
+    with_context(config["config"]) do
+      validate_type! config["config"], TrueClass, String
+    end
+  end
+end

test/commands/app_test.rb

@@ -300,6 +300,11 @@ class CommandsAppTest < ActiveSupport::TestCase
     assert_equal "ssh -t root@1.1.1.1 -p 2222 'ls'", new_command.run_over_ssh("ls", host: "1.1.1.1")
   end
 
+  test "run over ssh with custom config" do
+    @config[:ssh] = { "config" => "config/ssh config" }
+    assert_equal "ssh -F config/ssh\\ config -t root@1.1.1.1 -p 22 'ls'", new_command.run_over_ssh("ls", host: "1.1.1.1")
+  end
+
   test "run over ssh with no config" do
     @config[:ssh] = { "config" => false }
     assert_equal "ssh -F none -t root@1.1.1.1 -p 22 'ls'", new_command.run_over_ssh("ls", host: "1.1.1.1")

test/configuration/ssh_test.rb

@@ -42,4 +42,9 @@ class ConfigurationSshTest < ActiveSupport::TestCase
     config = Kamal::Configuration.new(@deploy.tap { |c| c.merge!(ssh: { "config" => false }) })
     assert_equal false, config.ssh.options[:config]
   end
+
+  test "ssh options with path to an ssh_config-file" do
+    config = Kamal::Configuration.new(@deploy.tap { |c| c.merge!(ssh: { "config" => "config/ssh_config" }) })
+    assert_equal "config/ssh_config", config.ssh.options[:config]
+  end
 end