This directory is populated with the built Cockpit packages when you run Webpack:
$ webpack
You can link this directory into your home directory and have Cockpit on your local machine use the built packages:
$ mkdir -p ~/.local/share/
$ ln -s $(pwd)/dist ~/.local/share/cockpit
Make sure to log into Cockpit as the same user on the same local machine as you are running the commands above.
If you want to setup automatic syncing as you edit javascript files you can:
$ webpack --progress --colors --watch