You can now run Docker images in Termux with Udocker/proot[0], the disk IO can be a bottleneck for large databases when using proot.

Tailscale works with "--tun=userspace-networking" [1].

I had it running on an old phone as a Frigate server with a solar powerbank in remote area, using the 4G as a failover. The uptime is almost a week without solar. Attiny hooked to the power button and a photodiode on the phone flash [2] (blink per minute) used as a watchdog for shutdowns/hangs to hardware reset. The button cap is removed without disassembling the phone.

Old phones are still more efficient than most off the shelf SBCs, especially under load. ~3W compared to 12W with a Pi5 in the same performance ballpark.

[0]: https://github.com/George-Seven/Termux-Udocker https://github.com/indigo-dc/udocker

[1]: https://tailscale.com/kb/1112/userspace-networking

[2]: https://wiki.termux.com/wiki/Termux-torch