Will pass through the hostname to any snapshots set up.
The hostname is _not_ derived from the random docker container string
but instead takes the name of the _host_ on which docker is running
(from ansible facts).
The hostname in combination with the tag should point to the correct
host -> stack which is being backed up.
Notifies double for each prune/check run which may need to be fixed.
Also custom notification contents cannot currently be passed.
Lastly, we should put identifying information into the notification body
(such as the hostname/container name for which the notification is
relevant).
Adapted from cloudserve-infrastructure, implements a backup stack using
restic. The actual backups have to be implemented by individual other
roles but this sets up initialization, pruning and checking of a repository.
