Add ERB eval so we can use credentials

lib/mrsk/configuration.rb

@@ -1,11 +1,12 @@
 require "active_support/ordered_options"
+require "erb"
 
 class Mrsk::Configuration
   delegate :service, :image, :env, :registry, :ssh_user, to: :config, allow_nil: true
 
   def self.load_file(file)
     if file.exist?
-      new YAML.load_file(file).symbolize_keys
+      new YAML.load(ERB.new(IO.read(file)).result).symbolize_keys
     else
       raise "Configuration file not found in #{file}"
     end