Problem
Dockerized app (in this case: RocketChat) loosing LDAP connection after reboot.
When installed, the App is just running fine with no errors. Just after the member server has been rebooted, the app looses it’s connection to the ldap server and therefore all user information.
Investigation
In order to access the app from Internet during the setup the external hostname was used to configure the app. This hostname was internally CNAMEd the internal (private) IP address of the server.
After a reboot the docker app used external nameservers and therefore was not able to resolve the hostname any more which lead to a failing ldap connection.
Solution
Configure the official hostname to resolve to the same (public) IP address no matter if the query sources internal or external.