add `-documentRoot ...` argument to gitlab-workhorse start as upstream
started to do:

    https://gitlab.com/gitlab-org/omnibus-gitlab/commit/d769f751

( gitlab-workhorse now needs to know where document root is as it now
  serves all content - e.g. static files too

  this option was introduced here:

    https://gitlab.com/gitlab-org/gitlab-workhorse/commit/1a7009e4  )

/cc @kazuhiko, @jerome