`FROM`: show progress even if nothing needs to be pulled

[qwofford]

For very large containers, the "FROM" stage can take a long time. If the container does not exist in CH_IMAGE_STORAGE, then charliecloud will report about the pull operation immediately, giving the user feedback to confirm progress. If the image does exist in CH_IMAGE_STORAGE storage, however, no output is given and when the associated copy operation takes a long time, it's tempting to assume nothing is happening, cancel the build, clear storage, and start from scratch.

Adding some verbosity to the copy operation in the above circumstance would give the me confidence that something is happening, and that I should just be patient.

[j-ogas]

I am curious how #1255 will impact this issue. Instead of copying the image it would be a checkout followed by a traversal to handle certain problem files, e.g., hard link etc.