AFAIK, image digest is a hash of image's manifest body.
When I pull busybox image from docker hub, and push it to my private registry, the digests get different.
$ docker pull busybox
...
Digest: sha256:2605a2c4875ce5eb27a9f7403263190cd1af31e48a2044d400320548356251c4
Status: Downloaded newer image for busybox:latest
$ docker tag busybox myregistry/busybox
$ docker push myregistry/busybox
...
08c2295a7fa5: Pushed
latest: digest: sha256:8573b4a813d7b90ef3876c6bec33db1272c02f0f90c406b25a5f9729169548ac size: 527
$ docker images --digests
myregistry/busybox latest sha256:8573b4a813d7b90ef3876c6bec33db1272c02f0f90c406b25a5f9729169548ac efe10ee6727f 2 weeks ago 1.13MB
busybox latest sha256:2605a2c4875ce5eb27a9f7403263190cd1af31e48a2044d400320548356251c4 efe10ee6727f 2 weeks ago 1.13MB
The images are not changed at all, and the image ids are same as each other.
But why image digests get different?
Updated:
Interestingly, the digest from another private registry is exactly same with the digest by my private registry.
$ docker image inspect efe10ee6727f
...
"RepoDigests": [
"myregistry/busybox@sha256:8573b4a813d7b90ef3876c6bec33db1272c02f0f90c406b25a5f9729169548ac",
"busybox@sha256:2605a2c4875ce5eb27a9f7403263190cd1af31e48a2044d400320548356251c4",
"anotherregistry/busybox@sha256:8573b4a813d7b90ef3876c6bec33db1272c02f0f90c406b25a5f9729169548ac"
],