Add ability to alias secrets for tags

Aliasing for secrets was introduced in #1439, but only supported
"top-level" secrets. This adds support for aliasing/mapping secrets
for tags.

lib/kamal/configuration/docs/env.yml

@@ -65,6 +65,13 @@ env:
 # MAIN_DB_PASSWORD=$(kamal secrets extract MAIN_DB_PASSWORD $SECRETS)
 # SECONDARY_DB_PASSWORD=$(kamal secrets extract SECONDARY_DB_PASSWORD $SECRETS)
 # ```
+env:
+  secret:
+    - DB_PASSWORD:MAIN_DB_PASSWORD
+  tags:
+    secondary_db:
+      secret:
+        - DB_PASSWORD:SECONDARY_DB_PASSWORD
 accessories:
   main_db_accessory:
     env:

lib/kamal/configuration/env.rb

@@ -1,7 +1,7 @@
 class Kamal::Configuration::Env
   include Kamal::Configuration::Validation
 
-  attr_reader :context, :secrets
+  attr_reader :context
   attr_reader :clear, :secret_keys
   delegate :argumentize, to: Kamal::Utils
 
@@ -11,27 +11,37 @@ class Kamal::Configuration::Env
     @secret_keys = config.fetch("secret", [])
     @context = context
     validate! config, context: context, with: Kamal::Configuration::Validator::Env
+    @secret_map = build_secret_map(@secret_keys)
   end
 
   def clear_args
     argumentize("--env", clear)
   end
 
+  def secrets
+    @resolved_secrets ||= resolve_secrets
+  end
+
   def secrets_io
-    Kamal::EnvFile.new(secrets_hash).to_io
+    Kamal::EnvFile.new(secrets).to_io
   end
 
   def merge(other)
     self.class.new \
       config: { "clear" => clear.merge(other.clear), "secret" => secret_keys | other.secret_keys },
-      secrets: secrets
+      secrets: @secrets
   end
 
   private
-    def secrets_hash
-      secret_keys.to_h do |key|
-        key_name, key_aliased_to = key.split(":")
-        [ key_name, secrets[key_aliased_to || key_name] ]
+    def build_secret_map(secret_keys)
+      Array(secret_keys).to_h do |key|
+        key_name, key_aliased_to = key.split(":", 2)
+        key_aliased_to ||= key_name
+        [ key_name, key_aliased_to ]
       end
     end
+
+    def resolve_secrets
+      @secret_map.transform_values { |secret_key| @secrets[secret_key] }
+    end
 end