This part could be made a little cleaner by just raising ``DockerTestFail()``.  Alternatively, always log the debug message and stick the conditional into ``self.failif()``.  Also, for negative tests, it's good to be clear about expected failures and unexpected successes, so how about:

    self.logdebug("Verifying expected '%s' is in stderr", searched_info)
    self.failif(status == 0, "Unexpected command success: %s" % self.stuff['cmdresult'])