Improve CI tests for building and running the container

[chrztoph]

Currently only the container is built and it is checked if the frontend can be built.
New tests should be added to check for the following:

• Check if run and build time env variables work by checking if they are available in the frontend
• Check if the app can be hosted at / and a subpath like /cms
• Check if the third-party-licenses.txt file is accessible

[gottfired]

License file check done.