resume pulling the layer on disconnect

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
pull only busybox:latest

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
fix busybox image detection

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6165 from vieux/update_tests

Update tests regarding busybox & inspect
switch skipping from error to debug

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6166 from vieux/debug_skipping

switch skipping from error to debug
Merge pull request #5848 from unclejack/resumable_pulls

resume pulling the layer on disconnect