util,arch-gcn3: Fixing gcn-gpu Dockerfile to v21-0 bucket

Was previously pointing towards the "develop" bucket which is subject to
change over time.

Change-Id: I769d8ac5dc2348e5a6477d75d883cc2a3add7bd3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45146
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed