All packages, except for Tini have been added to termux-root. To install them, simply pkg install root-repo && pkg install docker. This will install the whole docker suite, left only Tini to be compiled manually.
You will need something from your Stock ROM first. Get them all and Try to modify it using the procedure.
Note: This guide will be helpful for you if you have older mediatek devices (android-5.1 or android-6.0), but up-to-date devices can also work.
Update: This guide had a few typos and derps (Oops..), but I've tried to make it free of those now (Dated November 18, 2020)
Bonus: You can also use this tool by @SebaUbuntu, @yshalsager and @mauronofrio to auto generate the twrp tree. The tool is best if your device runs on android-9.0. The tools is still not 100% compatible to all devices, but I'll still recommend it and give it 8.5/10 in the scale of varsatility.
You will need something from your Stock ROM first. Get them all and Try to modify it using the procedure.