Add system startup snippets

Signed-off-by: Shin'ya Minazuki <shinyoukai@laidback.moe>

system-startup/gatus.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=Developer-oriented health dashboard
+Documentation=https://gatus.io
+After=network-online.target
+After=postgresql.service
+
+[Service]
+Type=simple
+User=gatus
+Group=gatus
+WorkingDirectory=/etc/gatus
+Restart=on-failure
+ExecStart=/usr/local/bin/gatus
+Environment=GATUS_CONFIG_PATH=/usr/local/etc/gatus.yml
+
+[Install]
+WantedBy=multi-user.target