Tonis Tiigi eefa8188e1 build: add experimental support for print flag
Print flag can be used to make additional information
requests about the build and print their results.

Currently Dockerfile supports: outline, targets, subrequests.describe

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
..
config refactor accessing registry configs via drivers
limited imagetools inspect: add --format flag
retryhandler build: add experimental support for print flag