Integrationstests erstellen

Related issues

Closes #57

Acceptance criteria

• Implemented code changes don't diverge from the related issue.
• Documentation reflects the status quo after merge.
• Version number is incremented according to SemVer. (Snapshot versions are not merged/released.)
• Only released versions of dependencies are used (i.e. no snapshot versions).
• Pipeline still works.
• GitLab issue is up-to-date.

If necessary, the following sections have been updated:

• Swagger
• Readme.md
• Documentation

Proposed squash commit message

Integrationstests für Basisfunktionen erstellen

• Add keycloak testcontainer and database config files for the tests.
• Add tests for the basic endpoints for the ecmr controller, group controller,
• user controller and template controller.