Thin clients off eBay. I picked up a Dell Wyse with 8gb memory, 4 cores, 16g emmc, and a 256G M.2 SSD for about $40. Includes the case, power supply, power button, etc. Still uses very little power. Install the x86_64 version of dietPi on that and it's been Rock solid running my docker projects.
Also picked up and HP T620 with similar specs. Haven't started using it yet but I expect similar results.
Much better deal than RPi and for most use cases equal or better able to do the job.
If you deployed with docker composr you just change the image and hit redeploy. Super simple.