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/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index 3de35cc..a22843d 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -690,7 +690,7 @@
 elif arch == "x86_64":
     docker_base = ""
 elif arch == "aarch64":
-    docker_base = ""
+    docker_base = "arm64v8/"
 else:
     print(
         f"Unsupported system architecture({arch}) found for docker image",