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>
diff --git a/util/dockerfiles/gcn-gpu/Dockerfile b/util/dockerfiles/gcn-gpu/Dockerfile
index e5683ab..2f5d1b4 100644
--- a/util/dockerfiles/gcn-gpu/Dockerfile
+++ b/util/dockerfiles/gcn-gpu/Dockerfile
@@ -52,7 +52,7 @@
 RUN python3 get-pip.py
 RUN pip install -U setuptools scons==3.1.2 six
 
-ARG gem5_dist=http://dist.gem5.org/dist/develop
+ARG gem5_dist=http://dist.gem5.org/dist/v21-0
 
 # Install ROCm 1.6 binaries
 RUN wget -qO- ${gem5_dist}/apt_1.6.4.tar.bz2 \