util-docker: Fix the gpu-fs docker image.

This was broken, it was actually building he gcn-gpu image, not the
gpu-fs image. This patch fixes this error.

Change-Id: I2d8ca0ea6584d059ddb6c9084de2b3075fb59723
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60532
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

util/dockerfiles/docker-compose.yaml

@@ -8,7 +8,7 @@ services:
         image: gcr.io/gem5-test/gcn-gpu
     gpu-fs:
         build:
-            context: gcn-gpu
+            context: gpu-fs
             dockerfile: Dockerfile
         image: gcr.io/gem5-test/gpu-fs
     sst: