The installation of GitLab on Synology uses
localhost as a default value for external url. This may lead to some problems when accessing GitLab over another IP or host name. In my case, this lead to missing icons and a non functional WebIDE. An inspection of the html page shows, that some resources are requested over
http://localhost/... which leads to 404 errors for those resources.
Since the GitLab container on Synology is not based on the omnibus package, you can not use directly
/etc/gitlab/gitlab.rb. If you want to change the url you can do it by changing the docker environment parameter