Hi,
in my migration process from Raspberry Pi4 to Synology DS918+ as Docker install I managed to port everything working so far. Home Assitant (Docker), Mosquitto Broker (Docker) and Conbee II via USB (for Zigbee) utilizing DeConz (Docker). Octoprint is perfectly integrated as well as Wake On Lan.
The only thing missing is RPC Shutdown. Since I do not have the Add-On Store (no supervisor) what would be the best / cleanest / simplest … 
… option to reactive that function from HA?
Why no supervisor?
And if you can get access to the underlying OS, then command line calls can be made to to do RPC actions.
As far as my research went … the supervisor is not support anymore in DSM7 when installing as docker. I do not want to install a virtual machine.
Hmm, you should be able to get it installed somehow anyway.
It will still not be supported as such, but you will get lots of the benefits, like HACS and so on.
Does this guide make sense to you?
Supervisor was never supported on DSM 6 in Docker - nothing changed with DSM 7.
I was aiming for a supported supervisor, but a working supervisor. 
Not explicitly in DSM7 … I ment the previous available install method as you also explained (Home Assistant How To - workaround for Hassio on Synology - YouTube).
Which is now deprecated …
Yes, the only officially supported method for Synology is VMM - hass.io package (although Docker version) was never considered Docker install TBH - it was using same method of installation that HA OS is using. And I know it’s strange, as supported version is also Docker version but… 
How would I do that … in a docker HA setup then? Sending the RPC shutdown command via Home Assistant through Synology…? Or use another RPC Shutdown docker …
So I got it working using MQTT/IOTLink Software (IOT Link / IOT Link Service · GitLab) installed on the Windows Machine.
The shutdown command also works before login which was important as well. Home Assistant with MQTT discovery detects the Machine after setup … 