[Arm-dev] Official docker containers are now available for AArch64/ARM64

As of the release of 7.4.1708, the official centos docker containers are now available via multiarch support from the docker hub. For those of you who have been using the centos/aarch64 namespaced base container, you can now use 'FROM centos' as the proper method.

I'll keep the centos/aarch64 container around for a little while longer to give people time to migrate.

TL;DR: 'docker pull centos' now does the right thing, and provides the image appropriate for your architecture.