Set default hostname to 'localhost' for webpackServer

[mradenovic]

When a hostname is not provided, a user is shown a message The app is running at HTTP://localhost:3000/. This, however, is not always correct. If the development is done within the crostini container, the default hostname will be http://penguin.linux.test:3000/.

Setting the default hostname to localhost makes everything work out of the box.