We're starting it with `--user` and dropped capabilities, after all.

Hopefully fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2188

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

This commit adds a 'matrix-ntfy' role that runs Ntfy server in Docker with
simple configuration, and plumbing to add the role to the playbook.

TODO: documentation, self-check, database persistence.