Evaluate cargo-wharf

[Shnatsel]

This project is using Docker to run sandboxed builds: https://www.reddit.com/r/rust/comments/dti647/announcing_cargowharf_docker_image_builder_for/

Apparently the sandboxing backends are also pluggable.

[tarcieri]

This looks neat! I played around with rustwide a bit (#5) and it seems like it might be interesting for reproducible builds but is less interesting for this particular purpose, so I'm down to check this out instead.