Added the suggested changes in the comments of the assignment.

src/main/docker/sonar/docker-compose.yml

src/main/java/de/rwth/swc/sqa/Application.java

src/main/java/de/rwth/swc/sqa/DataService.java

src/main/java/de/rwth/swc/sqa/api/CustomerController.java

src/main/java/de/rwth/swc/sqa/api/TicketController.java

src/test/java/de/rwth/swc/sqa/CustomerTest.java

src/test/java/de/rwth/swc/sqa/TicketTest.java