Use lastest 1.xx Rust version in Docker build

With the previous Rust version 1.61, the build fails with

> error[E0658]: use of unstable library feature 'scoped_threads'

This has been "stabilized" in Rust 1.63. To avoid the hassle
of manually updating the version, we can just default to the
latest minor release.

Dockerfile

@@ -1,5 +1,5 @@
 # See https://github.com/LukeMathWalker/cargo-chef
-ARG RUST_VERSION=1.61-buster
+ARG RUST_VERSION=1-buster
 FROM rust:${RUST_VERSION} as planner
 WORKDIR /scryer-prolog
 RUN cargo install cargo-chef