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/qemu-build.sh b/qemu-build.sh
index 6005128..6a60abd 100755
--- a/qemu-build.sh
+++ b/qemu-build.sh
@@ -57,7 +57,7 @@
     DOCKER_BASE=""
     ;;
   "aarch64")
-    DOCKER_BASE=""
+    DOCKER_BASE="arm64v8/"
     ;;
   *)
     echo "Unsupported system architecture(${ARCH}) found for docker image"