Docker BuildX for parallel builds and conda package cache

[scottyhq]

In the interest of simplifying and speeding up building an increasing number of images I started experimenting with Docker BuildX . One unanticipated result is the possibility of doing away with ONBUILD, which is a bit magical and confusing, and instead using a single pangeo Dockerfile with remote contexts. Discussed a bit with @yuvipanda at scipy2022. Lots to explore, so in the interest of not disrupting the currently functioning setup in this repository I'd welcome input over at https://github.com/scottyhq/pangeo-buildx

See also new Dockerfile syntax and buildkit options like (copy --link)
https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/reference.md#copy---link