Having used Docker. for over 5 months I can safely say that Docker is exceptional in Operations.
Divergence. Here are two ways your servers can Diverge from your expectations.
Someone logged in and modified a file that is not controlled by Chef. This file could be a critical file or innocent, the problem could be that it was not done in a way that could be repeated easily (or distributed to all servers).
Having Unattended Upgrades. configured and having it fail on some servers
The problem I have described here with Divergence is that Chef cannot revert a change to a file it is not in control of. Things that happen outside of Chef's control create results that cannot are not predictable.
Speed of Delivery
General reduction of complexity to deliver something. Breaking a large brittle product into smaller components
Shipping a product in a working state.
For example you can take a working image from 4 months ago and pull it down on a new server and t will just work
Docker by itself is powerful but without any automation or orchestration it can be the opposite.