Set absolute root path for gitlab-shell

--- a/lib/gitlab_init.rb
+++ b/lib/gitlab_init.rb
@@ -1,3 +1,7 @@
-ROOT_PATH = File.expand_path('..', __dir__)
+if ENV['SHELL_ROOT_PATH'].nil? || ENV['SHELL_ROOT_PATH'].empty?
+  ROOT_PATH = '/usr/share/gitlab-shell'
+else
+  ROOT_PATH = ENV['SHELL_ROOT_PATH']
+end
 
 require_relative 'gitlab_config'