notify-shutdown: Send a message on shutdown

Since Fedora CoreOS machines tend to reboot at seemingly random times
to apply updates, it would be nice to get a notification when they go
down.

notify-shutdown.service

@@ -0,0 +1,38 @@
+[Unit]
+Description=Send notification on machine shutdown
+RefuseManualStop=yes
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStop=/usr/bin/curl -d '%H is going down' https://ntfy.pyrocufflink.blue/alerts
+DynamicUser=yes
+CapabilityBoundingSet=
+DeviceAllow=
+DevicePolicy=closed
+LockPersonality=yes
+MemoryDenyWriteExecute=yes
+NoNewPrivileges=yes
+PrivateDevices=yes
+PrivateUsers=yes
+PrivateTmp=yes
+ProcSubset=pid
+ProtectClock=yes
+ProtectControlGroups=yes
+ProtectHome=yes
+ProtectHostname=yes
+ProtectKernelLogs=yes
+ProtectKernelModules=yes
+ProtectKernelTunables=yes
+ProtectProc=invisible
+ProtectSystem=strict
+RestrictAddressFamilies=AF_INET AF_INET6
+RestrictNamespaces=yes
+RestrictRealtime=yes
+RestrictSUIDSGID=yes
+SystemCallArchitectures=native
+SystemCallFilter=@system-service
+SystemCallFilter=~@privileged @resources
+
+[Install]
+WantedBy=multi-user.target

notify-shutdown.yaml

@@ -0,0 +1,14 @@
+variant: fcos
+version: 1.4.0
+
+storage:
+  files:
+  - path: /etc/systemd/system/notify-shutdown.service
+    mode: 0644
+    contents:
+      local: notify-shutdown.service
+
+systemd:
+  units:
+  - name: notify-shutdown.service
+    enabled: true