Add ntfy role

Installs and configures the ntfysh server to enable notifications.

roles/ntfy/README.md

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+# ntfy
+
+A self-hosted notifications service.
+
+Can take messages sent to the server through simple POST requests on specific topics and
+blasts them out to any subscribed receiver on Android, Web, Commandline, or even in other applications.
+
+Thus can function as a simple cross-platform push message service that fits very well into unix workflows.
+
+## Defaults
+
+```
+ntfy_upstream_file_dir: "{{ docker_stack_files_dir }}/{{ stack_name }}"
+```
+
+The on-target directory where the proxy configuration file should be stashed.
+
+```
+ntfy_use_https: true
+```
+
+Whether the service should be reachable through http (port 80) or through https (port 443) and provision an https certificate. 
+Usually you will want this to stay `true`,
+especially on the public facing web.
+
+```
+ntfy_version: latest
+```
+
+The docker image version to be used in stack creation.
+
+```
+subdomain_alias: push
+```
+
+If the deployed container should be served over a uri that is not the stack name. 
+By default, it will be set to `push.yourdomain.com` - 
+if this option is not set it will be served on `ntfy.yourdomain.com` instead.
+
+The individual `ntfy` options to be changed are very well described on
+[the ntfy documentation](https://ntfy.sh/docs/config/).
+Together with the default variables for this role it should be easy to find a good pair of settings.