Enclaver should support "cross-compiling" images for other architectures

[russellhaering]

Enclaver should support building images for architectures other than the local one.

I've started work on this here, but am putting it on hold due to trouble making image resolution work.

Currently, the first step "enclaver build" is image resolution, where we take various image identifiers (source image, odyn image, wrapper-base image, etc) and resolve them to a specific image ID. This is all orchestrated through the Docker daemon - enclaver never talks to an image registry directly.

However, the Docker Engine API doesn't seem to have good support for listing or interacting with non-native images.

An alternative option might be to attempt to interact with registries directly, but:

1. We'd need to be able to authenticate to the registry (or possibly use this API?).
2. This would break the ability to build from a local source image - ie, source images would always have to be pushed to a registry.

Interestingly, Docker itself seems to be moving away from supporting building from local source images with buildx (for example) - but this seems like a pretty critical requirement for most CI pipelines.