Tagflip is an online tagging tool similar to BRAT or WebAnno.
While BRAT is rather lightweight, it runs only on UNIX-like environments and is a little outdated (Python 2.5 and CGI interface). WebAnno is actively maintained, but it uses a UI based on Whicket and the code is quite complex.
Tagflip consists of a Node.js backend (which can be deployed as a docker container, if you like) and a lightweight React frontend.
This npm package contains the code shared between backend and frontend.