Be really careful when building images that require secrets for build configuration. Secrets can be passed in as build args, but you MUST UNSET THEM IN THE DOCKERFILE and then repass them in as environment variables at runtime (or else you are leaking your secrets with your image).
Also, image != container. Image is the thing you publish to a registry (e.g. dockerhub). Container is an instance of an image.
Thanks for sharing! I will need to look deeper into build kit. Containers aren't my main artifacts, unfortunately, so I am still building them the ways of old, sounds like.