Allow querying for current pulls

[mark-dr]

Description

Currently, the engine does not have a way to list the pulls that are currently in progress. If your code didn't initiate a pull, the best you can do is be notified when it completes, by listening for a pull event on /events.

This creates challenges for UI apps. For instance, suppose I have a list of images in a dashboard, and I use an integrated terminal to run docker pull. Ideally the user would see the in-progress pull in the image list, and if they went to the details page for an image, they'd be informed that a corresponding pull is in progress. There's no practical way to achieve that currently.

Proposal

• Either:
  • Add a parameter to /images/json (strawman suggestion: include-pulls). If this is set to true, the resulting list of images should contain new entries representing pulls in progress. They would have a different shape to the other entries, so maybe it makes more sense to...
  • Add a new route (strawman suggestion: /images/pulling/json) listing all in-progress pulls, potentially also with progress information.
• Add new events (either to /events or some new endpoint) that fire when a pull starts, or when a pull makes progress. The latter would be similar to the progress events emitted by /images/create when pulling an image.

Related issues

• Cancelling pulls #45433