Change watchtower image used in docker deployments

With the release of Docker v29, the Watchtower image used to automatically update the Outline server will break with the following error: ``` time="2025-11-28T15:27:31Z" level=error msg="Error response from daemon: client version 1.25 is too old. Minimum supported API version is 1.44, please upgrade your client to a newer version" time="2025-11-28T15:27:31Z" level=info msg="Waiting for the notification goroutine to finish" notify=no ``` An [issue](https://github.com/containrrr/watchtower/issues/2122) has been created in the Watchtower repository, but since the project appears to be inactive, I don’t expect it to be fixed soon. The issue also links to a [fork](https://github.com/nicholas-fedor/watchtower) of Watchtower, which works with Docker v29 in my own testing. A quick search shows that only [this line](https://github.com/Jigsaw-Code/outline-server/blob/26803710c9921c91b4cf1435ed15c67cfbf9ca90/src/server_manager/install_scripts/install_server.sh#L379) may need to be changed, although I might be wrong in that assumption.