CI (`TestPush`): Image push test fails with "content digest ... not found"

[ChengyuZhu6]

Intermittently, CI test cmd/nerdctl/image TestPush fails during nerdctl push to the local registry: the push exits with code 1 and reports the fatal error “content digest ... not found.”

https://github.com/containerd/nerdctl/actions/runs/17036605946/job/48290458405?pr=4456
https://github.com/containerd/nerdctl/actions/runs/17036605946/job/48290458392?pr=4456
https://github.com/containerd/nerdctl/actions/runs/17035364226/job/48286896972?pr=4467

[AkihiroSuda]

Likely relevant:

• content digest not found: platform-reduced push, follow-up to #3425 #3509