clean all application images built during tests

[zmiklank]

• introducing init function for ct tests

The changes in all containers which will be needed can be seen in this exemplary PR: https://github.com/sclorg/nginx-container/pull/224

Edit: added commit for preserving backwards compatibility.

[zmiklank]

@phracek just for your info - this is still a work in progress - the PR is marked as draft, as something is not working as it should.

[zmiklank]

[test-all]