Oh, gotcha. You have CI for multiple architectures already and you want to add arm64? I was thinking it was brand new code you wanted to write, but I get why you wouldn't want to rewrite everything.
Sadly we adopted CircleCI early on and make heavy use of their support for multiple containers - if you specify a list of containers in a Docker job, it will execute the tests in the first container and connect the other containers as "data" containers (think redis, mysql et c) for use in tests.