Spammy output when many images are built at once

[timothyb89]

Example: https://travis-ci.org/monasca/monasca-docker/builds/265038429

Multiple tqdm status bars seem to get really spammy when there's lots of status bar lines. This looks okay in a real terminal but is actually causing travis to kill jobs when log output is too large (> 4MB).

I think the 'correct' workaround is to just periodically display a simple status message when outputting to a dumb terminal - tqdm kind of does this already (poorly) but some of dbuild's hacks break it even more.

Another option: add an option to write build logs to a file for each container. We can upload them somewhere (maybe a Google Cloud Storage bucket) and avoid turning on dbuild's debug mode and generating output spam in the first place.