Deploy multi-architecture (amd+arm) docker testing images

Docker's buildx supports building multi-architecture images.

This can be used to deploy our testing images: apache/cassandra-testing-ubuntu2004-java11 and apache/cassandra-testing-ubuntu2004-java11-w-dependencies; for both linux/amd64 and linux/arm64 architectures.

(Up until now ci-cassandra.a.o is having to manually local build these images during each test run.)