Picked up a couple of HPE T520 Thin Clients for $10 a piece.
Loader: Jun's Loader v1.04b for DSM 6.2 on a usb stick
DSM: DSM 6.2.2-24922 Update 5 DS918+ installed on internal M.2 8G SSD
Tested Working with Docker, Plex, DS Video
Also tested working with 1.02b loader for 3615x running DSM 6.1.
Cheapest hardware I have found.
Cheers