Optimize Rust build in Docker

This is a docker file I pushed that relies on docker caching to optimize builds:

https://github.com/testground/testground/pull/1327/files#diff-ea696a48937940f71a6d8064e6ca41eb74b1ff17bc0e18d475c83cadaf77f1ce


Laurent Senta

I wrote software for large distributed systems, web applications, and even robots. Now I focus on decentralization, overly-engineered software, and frugal innovation. I help companies around the world build products through SingularGarden.