Support sshkit.default_env

Enables deploy to MacOS hosts.

Will still need further configuration though:
- `default_env` needs to be set to a PATH that contains docker and coreutils `cp`
- desktop docker config needs to have no `credsStore`
- shell needs to be changed to `bash`

All this can be done by the user though, whereas passing on `default_env` to SSHKit needed to happen on Kamal.

cc @dhh, @HLFH

See #432 (comment) for details.

lib/kamal/commander.rb

@@ -168,6 +168,7 @@ def configure_sshkit_with(config)
       end
       SSHKit.config.command_map[:docker] = "docker" # No need to use /usr/bin/env, just clogs up the logs
       SSHKit.config.output_verbosity = verbosity
+      SSHKit.config.default_env = config.sshkit.default_env
     end
 
     def specifics

lib/kamal/configuration/docs/sshkit.yml

@@ -21,3 +21,10 @@ sshkit:
   # Kamal sets a long idle timeout of 900 seconds on connections to try to avoid
   # re-connection storms after an idle period, such as building an image or waiting for CI.
   pool_idle_timeout: 300
+
+  # Default Env
+  #
+  # SSHKit sessions do not inherit the host PATH value. If you need to set custom env vars on 
+  # a SSHKit session, like PATH or DOCKER_DEFAULT_PLATFORM you can map them here.
+  default_env:
+    path: /usr/local/bin:$PATH"

lib/kamal/configuration/sshkit.rb

@@ -5,7 +5,7 @@ class Kamal::Configuration::Sshkit
 
   def initialize(config:)
     @sshkit_config = config.raw_config.sshkit || {}
-    validate! sshkit_config
+    validate! sshkit_config, with: Kamal::Configuration::Validator::Sshkit
   end
 
   def max_concurrent_starts
@@ -16,6 +16,10 @@ def pool_idle_timeout
     sshkit_config.fetch("pool_idle_timeout", 900)
   end
 
+  def default_env
+    sshkit_config.fetch("default_env", {}).transform_keys(&:to_sym)
+  end
+
   def to_h
     sshkit_config
   end

lib/kamal/configuration/validator/sshkit.rb

@@ -0,0 +1,9 @@
+class Kamal::Configuration::Validator::Sshkit < Kamal::Configuration::Validator
+  def validate!
+    validate_against_example! \
+      config.except("default_env"),
+      example.except("default_env")
+
+    validate_hash_of!(config["default_env"], String)
+  end
+end

test/configuration/sshkit_test.rb

@@ -25,4 +25,10 @@ class ConfigurationSshkitTest < ActiveSupport::TestCase
     @config = Kamal::Configuration.new(@deploy.tap { |c| c.merge!(sshkit: { "pool_idle_timeout" => 600 }) })
     assert_equal 600, @config.sshkit.pool_idle_timeout
   end
+
+  test "sshkit default env" do
+    assert_equal {}, @config.sshkit.default_env
+    @config = Kamal::Configuration.new(@deploy.tap { |c| c.merge!(sshkit: { "default_env" => {"path" => "/usr/local/bin:$PATH" } }) })
+    assert_equal {path: "/usr/local/bin:$PATH" }, @config.sshkit.default_env
+  end
 end