Publish linux/arm64 Docker container

[bluekeyes]

#556 enabled linux/arm64 builds, but only for distributions. The current version of Godel / distgo does not support building multi-architecture containers unless using an internal-only plugin. We'll need to wait for support in the open-source version to publish other container architectures, or temporarily stop using Godel for our Docker builds.

[ab77]

in the meantime: https://github.com/product-os/policy-bot/pkgs/container/policy-bot