multi-stage builds

[cyphar]

Currently we only support single-FROM builds. But in Docker 17.05 (moby/moby#31257), multi-stage builds are a thing and for compatibility we need to implement it too -- shouldn't be too hard we just need to support multiple base image sources. COPY --from will be a bit more painful.

[cyphar]

There's also named stages (moby/moby#32063).