added simple healthcheck route

app/controllers/healthcheck_controller.rb

@@ -0,0 +1,9 @@
+class HealthcheckController < ApplicationController
+  unloadable
+
+  skip_before_action :check_if_login_required
+
+  def index
+    render :layout => false, :action => 'index'
+  end
+end

app/views/healthcheck/index.html.erb

@@ -0,0 +1 @@
+OK

config/routes.rb

@@ -1,5 +1,6 @@
 if Redmine::Plugin.installed? :baupm_core
   RedmineApp::Application.routes.draw do
     get '/manifest.json', :controller => 'manifest', :action => 'index', :as => 'manifest_json'
+    get '/healthcheck', :controller => 'healthcheck', :action => 'index', :as => 'healthcheck'
   end
 end