ci: change to docker "bridge" network driver (#7003)
ci: change to docker "bridge" network driver (#7003)

This restores network isolation between the container and the docker
host, which should allow for using fixed ports from concurrent
containers without interference. The DNS cache problems alluded to
from #4288 appear to be a thing of the past.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp