Use .env file for GitLab

It is necessary to define the author of the commit, currently this information is hardcoded. Change it and use the information from the .env file. Important, since it can happen that some forgets to add this information, check if the values are empty. If so please raise an appropriate exception. In the future the information from the exceptions should be displayed in the frontend.

Furthermore, remove the credentials from the docker-compose and put it into the .env file. Change the code as well. Do the same check and raising exception like above.

Lastly put the .env file on the .gitignore so that no one can leak their credentials again. : - )