My preference is to use the build system to create built artifacts, and I consider the build/ directory to be a built artifact. Wrangling Git into doing the first fundamental build step is off, in my opinion.

However, if you disagree, my favorite "Git keep" filename is "README.md". Why is this otherwise empty directory here, how does it fit into my source tree, how is it populated, and so forth.

One of my pet peeves with the latest AI wave is the time we spend creating files to help AI coding agents, but don't give the same consideration to the humans who have to maintain and update our code.