Fix E2E test mysql docker cleanup

[FelberMartin]

Description

There seems to be a problem related to the docker files, where the mysql database is not reset properly. This causes the newly created courses by DashboardE2eTest to pile up and causes the server to take very long to return all the courses. This results in a timeout when calling /api/courses/for-dashboard.

This behaviour can be both observed locally and on the github runners.

Maybe related: Sometimes when running the E2E tests on github, they fail because of the mysql docker container complaining that port 3306 is already in use, even though we added an extra cleanup step to the e2e-test.yml job file that should kill everything still running on 3306.

Next steps

• Properly reset the mysql database
• Re-enable the DashboardE2eTest (disabled in Bugfix: Removed moderator rights to edit posts; added test #119)