We're holding back two things:
- Postgres 12 and up breaks `libpq-sys`.
- Ubuntu 20.04 breaks `sqlx`.
We intend to merge this branch as the main release.
This PR attempts to lay the groundwork to address
https://github.com/emk/rust-musl-builder/issues/96. In particular, we
install the Rust toolchain globally by abusing `rustup`.
We still preserve the legacy `rust` user but we now encourage derived
Dockerfiles to consider `USER root`.
This PR also removes ARM support. We want to explore the idea of
supporting ARM more reliably using a separate image, but perhaps it
would be better to refer those users to one of the Rust
cross-compilation toolchains. See
https://github.com/emk/rust-musl-builder/issues/63 for discussion.
