Docker image containing rust and some preinstalled packages used in CI.

This image may be manually updated by running `./build.sh` if you are a member
of the [Solana Labs](https://hub.docker.com/u/solanalabs/) Docker Hub
organization, but it is also automatically updated periodically by
[this automation](https://buildkite.com/solana-labs/solana-ci-docker-rust).
