The 'Docker - Delete unused manifests and images' task performs a garbage collection of unreachable Docker image assets and removes them. The purpose of the "deploy offset in hours" setting in this task is to avoid a condition that could result in the deletion of Docker images that are being uploaded whilst the task is running.
When an image is pushed, Docker will first push the image layers, then its manifest, and finally the tag of the manifest. If there were no offset setting, it would be possible for the task to think that Docker image layers or manifests that are in the middle of being deployed were eligible for garbage collection because they are not yet referenced by any tag.
We recommend leaving that setting at its default value of 24 hours. That is a very conservative setting as a single image push should not ever take this long. Leaving it at the default value will ensure the task does not mistakenly garbage collect in-flight image uploads.