refactor(other): cache docker images in github e2e flow

Created by: mahula

Pullrequest

At the moment each e2e test job runs about 17 - 30 min. Main reason is building the SUT docker images (about 15 min) in each e2e test job.

Todo

• extract docker image building into a separate preparation job to build only once
• use Github Cache Action to store and acces the docker images
  • delete cache after test runs

Result

Now one preparation job builds images once. The e2e test jobs run 15 min less now. This way the used runners will be freed much earlier for other workflow jobs.