Refactor test containers

[8-bit-fox]

Description

This PR aims at removing the project code from the test images. Reasons have been explained in #2344 . Same as #2347 rebased to 2.6.0.

Additional information...

• Adds project code-free images
• Adds a docker-compose file for local test execution
• Adds an entrypoint (Code formerly part of the testing image)
• Opens possibility for:
  • providing test-base images via ghcr.io (eventually via a pipeline)
  • Running tests on Github Actions, while continuing to run on TravisCI
  • Runtime reduction (== less credits spent) as the containers would be built only once

[8-bit-fox]

Ah, ship... I wanted to make it a draft. @tbabej , are you allowed to convert it?

[tbabej]

@8-bit-fox Converted. Not sure if you see the conversion link, but this is where to find it: https://github.blog/changelog/2020-04-08-convert-pull-request-to-draft/