Replace deprecated containrrr/watchtower

[fortuna]

Watchtower is no longer maintained, we need to replace it.

We could fork the repository and build our own image. The Dockerfile is https://github.com/containrrr/watchtower/blob/main/dockerfiles/Dockerfile.dev-self-contained

Another option is to put the container update into the shabowbox image, so it updates itself. That would require access to the docker socket, with security implications.

[fortuna]

Another option is to use Podman's auto update: https://docs.podman.io/en/stable/markdown/podman-auto-update.1.html

That will require a migration to podman, which is ok. It would only apply to new installations anyway.

[fortuna]

If we change any image in Outline, it's an opportunity to introduce a redirector, so we can do migrations without disruption in the future.
Here is an example using HTTP redirects: https://httptoolkit.com/blog/docker-image-registry-facade/

One key point was to enable the redirect only for our organization, so it's not a general redirector.

/cc @ohnorobo