@dark_dragone not 100% sure but I think search domains is a client feature. This means that docker (the DNS client on behalf of the guac container) should be configured to have a dns search. See https://docs.docker.com/config/containers/container-networking/ (--dns-search). Maybe you can quickly try by editing /etc/systemd/system/docker.service.d/cloudron.conf and restarting docker.
I think you can also add it in your local machine's /etc/resolv.conf