Made rust version in Dockerfile static, this seems to have really fixed the cache issues

Dockerfile

@@ -1,17 +1,18 @@
 # See https://github.com/LukeMathWalker/cargo-chef
-FROM rust as planner
+ARG RUST_VERSION=1.52.1-buster
+FROM rust:${RUST_VERSION} as planner
 WORKDIR /scryer-prolog
 RUN cargo install cargo-chef 
 COPY . .
 RUN cargo chef prepare --recipe-path recipe.json
 
-FROM rust as cacher
+FROM rust:${RUST_VERSION} as cacher
 WORKDIR /scryer-prolog
 RUN cargo install cargo-chef
 COPY --from=planner /scryer-prolog/recipe.json recipe.json
 RUN cargo chef cook --release --recipe-path recipe.json
 
-FROM rust as builder
+FROM rust:${RUST_VERSION} as builder
 WORKDIR /scryer-prolog
 COPY . .
 # Copy over the cached dependencies