[SOLVED] What's the difference between a Docker image's Image ID and its Digest?

What's the difference between a Docker image's Image ID and its Digest?

This has been surprisingly confusing for me. I thought Docker's Image ID is its SHA256 hash. However, apparently the result from docker image ls --digests (listed under the column header DIGEST) is different from the IMAGE ID of that image.

For example

$ docker image ls --digests alpine
REPOSITORY  TAG     DIGEST                                                                   IMAGE ID      CREATED      SIZE
alpine      latest  sha256:769fddc7cc2f0a1c35abb2f91432e8beecf83916c421420e6a6da9f8975464b6  055936d39205  2 weeks ago  5.53MB

while

$ docker image ls --no-trunc
REPOSITORY  TAG     IMAGE ID                                                                 CREATED      SIZE
...                                                                                                       
alpine      latest  sha256:055936d3920576da37aa9bc460d70c5f212028bda1c08c0879aedf03d7a66ea1  2 weeks ago  5.53MB

Clearly

sha256:055936d3920576da37aa9bc460d70c5f212028bda1c08c0879aedf03d7a66ea1 (IMAGE ID) and
sha256:769fddc7cc2f0a1c35abb2f91432e8beecf83916c421420e6a6da9f8975464b6 (DIGEST) are not the same value. But why? What's the purpose of having two different sha256 hashes of the same image. How are they calculated, respectively?

I was confused by this when reading the book Docker Deep Dive, and I haven't been able to find a clear answer either in the book or online.

Solution

Thanks for michalk's comment. The short answer is:

The "digest" is a hash of the manifest, introduced in Docker registry v2.
The image ID is a hash of the local image JSON configuration.