By the time you get a raspi, power supply, case, external hdd, and sd card you're looking at over $200, at which point I'd say just get a refurbished Dell Optiplex. They're much more reliable and higher performance than a Pi and if something breaks its modular and you can replace individual components, unlike a Pi. The same Docker container for RasPi will run in Ubuntu on an Intel Dell Optiplex, too.