Build and test on arm64

We discovered that the `arm64` images were based on `amd64` base images (#78) that went undiscovered as
1. We weren't running tests on arm64 — I wasn't aware that Travis supported arm64.
2. I was performing manual tests using Docker Desktop for M1 and Minikube, and Docker Desktop installs `binfmt_exec` extensions to allow non-native images.

We're also hitting frequent rate limiting issues with Docker Hub on Travis. Since Github Actions supports builds in amd64 and arm64, so we should run tests there.