Set KAMAL_DESTINATION for dotenv parsing

lib/kamal/configuration/secrets.rb

@@ -1,24 +1,29 @@
 class Kamal::Configuration::Secrets
-  attr_reader :secret_files
+  attr_reader :secret_file, :destination
 
   def initialize(destination: nil)
-    @secret_files = \
+    @destination = destination
-      (destination ? [ ".kamal/secrets.#{destination}", ".kamal/secrets" ] : [ ".kamal/secrets" ])
+    @secret_file = (destination ? [ ".kamal/secrets.#{destination}", ".kamal/secrets" ] : [ ".kamal/secrets" ])
+      .find { |file| File.exist?(file) }
   end
 
   def [](key)
     @secrets ||= load
     @secrets.fetch(key)
   rescue KeyError
-    if secret_files.any?
+    if secret_file
-      raise Kamal::ConfigurationError, "Secret '#{key}' not found in #{secret_files.join(', ')}"
+      raise Kamal::ConfigurationError, "Secret '#{key}' not found in #{secret_file}"
     else
-      raise Kamal::ConfigurationError, "Secret '#{key}' not found, no secret files provided"
+      raise Kamal::ConfigurationError, "Secret '#{key}' not found, no secret file provided"
     end
   end
 
   private
     def load
-      secret_files.any? ? Dotenv.parse(*secret_files) : {}
+      original_env = ENV.to_hash
+      ENV["KAMAL_DESTINATION"] = destination if destination
+      secret_file ? Dotenv.parse(*secret_file) : {}
+    ensure
+      ENV.replace(original_env)
     end
 end