Rust Docker image documentation needs to be updated

[spencerbart]

The example code for the Rust Docker image documentation should be updated to something like this. The previous example is out of date.

FROM rust:1.64 AS builder
COPY . .
RUN cargo build --release

FROM debian:buster-slim
COPY --from=builder ./target/release/my-rust-app ./target/release/my-rust-app
CMD ["/target/release/my-rust-app"]

[tianon]

cc @sfackler (rust image maintainer)