docker/ci: Add Rust tools to Dockerfile
docker/ci: Add Rust tools to Dockerfile

Installs Rust nightly (locked to 2017-02-25 snapshot) via Rustup:

https://www.rustup.rs/

Also installs the following development tools:

- rustfmt: Lint syntax to ensure it follows standard style (ala gofmt)
- clippy: Lint for non-idiomatic Rust or other minor mistakes
- cargo-audit: Audit Cargo.lock for crates containing security
vulnerabilities

Closes #614

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain