`taskgraph load-task` should cache docker images

[matt-boris]

Currently, if you run, say taskgraph load-task Ww-q-IvVTfed8xepUhNWaw twice, you'll see:

2025-09-26 15:54:49,487 - INFO - Downloading from https://community-tc.services.mozilla.com/api/queue/v1/task/MUvi2npBRue4VmksTmsdEw/artifacts/public/image.tar.zst
Loaded image: ci:latest

each time in the logs. It would be useful to cache this downloaded image, as some of these images are large and this takes up a good portion of the time when running load-task.

Subsequent runs would be much faster and help the local debugging/development process.

[matt-boris]

Another instance would be making changes to an in-tree Dockerfile and testing against multiple different taskIDs. Each run of taskgraph load-task --image <in-tree image name> <taskID> rebuilds the same image.

[ahal]

Implementation note:

We probably need to extract the digest hashes that Taskgraph computes as part of the cached_task transforms. Then upon building the image, we apply a tag containing this hash to the image in the local registry. On subsequent calls to build_image, we then check whether an image with the same digest tag exists and return that if so.

We'll probably also want to add a --force flag to force a rebuild even if there's a locally cached option.