Missing .env file in development

The new plugin mechanism relies on the fact that the server distributes the env.js and artifacts.js files. These are loaded already in the index.html of our React application, and thus this file needs to know the server address beforehand. This is ensured by providing a .env file in the GUI build directory. This file is created in the docker image, but it is not automatically created in a development environment. To fix this, the setup_dev_env.sh script should be modified and the env file should be included in version control.