Describe the bug
When running yarn build, the command cp -r ./web-app/build/ ./builld copies the ./web-app/build to ./build/build folder.

I think the right command would be:

See https://askubuntu.com/a/86824

cp -r ./web-app/build/. ./build/ # notice the dot

Desktop (please complete the following information):

• Platform: Linux
• OS: Fedora
• Version 31