Improve before_script in docker-in-docker jobs

@hwtb2 proposed this improved version of before_script for docker-in-docker job

timeout 30 sh -c 'until docker info > /dev/null 2>&1; do sleep 1; done' \
        || { echo "Timed out waiting for dockerd"; exit 1; }

This is significantly better than original as it sets timeout and don't waste time if something is wrong with underlying service.