* feat: allow hostname override for email notifier
As it currently stands all notifiers utilise `os.Hostname` to populate their titles/subjects.
When utilising Docker with a bridged network if you set the hostname for a container to an external DNS hostname Docker's internal DNS resolver will override said hostname for all containers within the bridged network.
This change allows a user to specify what hostname should be represented in the email notifications without having to change the `os.Hostname`.
* feat: allow custom hostname for all notifiers
* docs: adjust notification hostname flag
This adds support for Gotify (https://gotify.net) notifications.
Work items:
* Two flags have been added to internal/flags/flags.go:
"notification-gotify-url" and "notification-gotify-token".
* A Gotify notification driver has been added in
notifications/gotify.go.
* "gotify" has been added to notification driver choices in
notifications/notifier.go.
* Docs have been updated
