Fix docker pulling image integration test
What does this MR do?
Updates the integration test checking if a specific type of docker image pull error is considered as a script failure.
Why was this MR needed?
The existing test was depending on a domain, which behavior was recently changed.
According to the discussion at !3060 (comment 637779734), the goal was not to cover all possible errors that we're trying to solve but to generally test the behavior.
We should be able to fix the test by using the example.com second-level domain. It's reserved by IANA for documentation purpose and - apart of example.com itself which resolves to a simple webpage informing about it's purpose - we can expect, that docker.repo.example.com will always fail the DNS resolution. Which should always fail the pull and therefore achieve what the test is trying to test.