aarch64 support missing

[mike503]

As of 9.18 there is support for aarch64 support (Graviton 2, ARM) - I do see that this project leverages newrelic-install to install the daemon, which also has a issue I've logged there[1], that blocks this one from being completed without a workaround. However, once that installer ships the aarch64 support, then this is likely a very minor change to the build pipeline to publish the image to docker hub.

1. Latest agent download does not include aarch64 support newrelic-php-agent#323