Enable docker secrets in the builder as a more secure alternative to build args.

lib/mrsk/commands/builder/base.rb

@@ -2,6 +2,7 @@ require "mrsk/commands/base"
 
 class Mrsk::Commands::Builder::Base < Mrsk::Commands::Base
   delegate :argumentize, to: Mrsk::Configuration
+  delegate :simple_secretize, to: Mrsk::Configuration
 
   def pull
     docker :pull, config.absolute_image
@@ -11,8 +12,16 @@ class Mrsk::Commands::Builder::Base < Mrsk::Commands::Base
     argumentize "--build-arg", args, redacted: true
   end
 
+  def build_secrets
+    simple_secretize "--secret", secrets, redacted: true
+  end
+
   private
     def args
       config.builder["args"] || {}
     end
+
+    def secrets
+      config.builder["secrets"] || {}
+    end
 end