Show progress bar when downloading Docker images

Created by: mrnugget

Not sure whether that fits your vision for how you want to show progress, @LawnGnome, but since the image pulling should be done once for all steps and not for each task/repository, I think it makes sense to pull it out.

Also tried to follow your convention of not mixing output into the Service but providing a progress callback.

Demo GIF

This uses some large images, so don't be surprised that it takes so long.