Show some sort of feedback when downloading docker images

[tfoote]

It can be confusing when rocker appears to be doing nothing, but it's actually downloading the docker images necessary to run.

Getting hooks into the actual download might be a challenge, but if we detect and prefetch the base images before hand we can provide feedback during that and not have to worry about getting the feedback from the build stage.

This likely will overlap with #26

Also make sure to cover intermediate/helper image fetching such as the python image used for the os detector. It's 900Mb which is bigger than I'd hoped.

[tfoote]

There was some of this available prior to #100 being merged, but it was pulling too much so relying on the internal pull mechanism is more efficient but less transparent.