docker-util: Update the gcn-gpu docker image Cloud Bucket

This is updated for the gem5 v21.1 release.

Change-Id: I84e663afdca41045a73e2fc25b87b53a6063202c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48663
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
1 file changed