Use native docker images for scripts on aarch64 Host OS

Commit 051b05b7 support aach64 Host OS to execute scripts. However, it
does not set DOCKER_BASE parameter which consequently uses x86_64 docker
images instead of arm64v8 image.
This commit sets DOCKER_BASE to arm64v8/ to use native docker images.

Tested:
1. Set environment variables appropriately and execute scripts. Ensure
all docker images loaded successfully and scripts can execute without
issue.

Signed-off-by: Thang Q. Nguyen <thang@os.amperecomputing.com>
Change-Id: I3c09d448dae17f290f2c2fd1d8116167510635b6
diff --git a/openpower-build-setup.sh b/openpower-build-setup.sh
index 1ca5f25..3d8bc73 100755
--- a/openpower-build-setup.sh
+++ b/openpower-build-setup.sh
@@ -36,7 +36,7 @@
         DOCKER_BASE=""
         ;;
     "aarch64")
-        DOCKER_BASE=""
+        DOCKER_BASE="arm64v8/"
         ;;
     *)
         echo "Unsupported system architecture(${ARCH}) found for docker image"