Docker remote repository digest pulls may fail when the cache period is set to zero

Docker remote repository digest pulls may fail when the cache period is set to zero.

To reproduce

1. Set up two Artifactory 5.10.3 instances
2. Create a local Docker repository in one and remote one (with the metadata cache period set to zero) in another
3. Upload an image to the local Docker repository
4. Point the remote repository to the local and
5. Attempt a digest pull several times from the remote (or use curl) and notice the intermittent 404's

Workaround

1. Set the metadata cache retrieval period (under the advanced setting of the remote Docker repository) to non zero (even 1 would work)