Docker 1.12 has a nice set of improvements that significantly improve the ability to administrate the Docker daemon. https://t.co/bXt4PwPXRr
— Kelsey Hightower (@kelseyhightower) 31. Juli 2016
On the Docker GitHub release/changelog page of v1.12.0 under the Runtime section the first point:
Add –live-restore daemon flag to keep containers running when daemon shuts down, and regain control on startup
#23213
This new option allows containers to keep running even when stopping, restarting or upgrading the Docker daemon.
TL;DR The --live-restore flag keeps containers running during a Docker daemon update.