This tutorial shows how to run docker natively on Android, without VMs and chroot. - docker.md

I think Docker opens up a lot of ways to repurpose an old phone and turn it into a home server. The advantage over using something like RPi is that you have a touchscreen built in, making it much easier to troubleshoot without having to plug in a monitor and a keyboard.

Given how abundant old phones are, seems like it would be cool to have an Android distro specifically designed for this use case. Especially if you could plug in a USB hub for stuff like external storage.