RFR (XS): 8221529: [TESTBUG] Docker tests use old/deprecated image on AArch64

[Nick Gasson]

Hi,

This is a follow on from 8221342 to update the default Docker image used 
on AArch64 from "aarch64/ubuntu" to "arm64v8/ubuntu". According to 
Docker Hub the former is deprecated and hasn't been updated since Ubuntu 
16.04. This causes symbol resolution failures if the JDK image being 
tested was built against a recent glibc.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221529
Webrev: http://cr.openjdk.java.net/~ngasson/8221529/webrev.0/

Tested using the runtime/containers/docker hotspot jtreg tests on 
AArch64 and x86.

Thanks,
Nick