Remove version constraints on container images (#549)

This fixes errors during the build of this image, occuring due to
non-existent version tags.

Dockerfile

@@ -1,20 +1,22 @@
-FROM rust:1.64-bullseye AS selene-builder
+ARG RUST_VERSION="1"
+
+FROM rust:${RUST_VERSION}-bullseye AS selene-builder
 RUN apt-get update && \
     apt-get upgrade -y && \
     apt-get install g++ && \
     cargo install --branch main --git https://github.com/Kampfkarren/selene selene
 
-FROM rust:1.64-bullseye AS selene-light-builder
+FROM rust:${RUST_VERSION}-bullseye AS selene-light-builder
 RUN apt-get update && \
     apt-get upgrade -y && \
     apt-get install g++ && \
     cargo install --no-default-features --branch main --git https://github.com/Kampfkarren/selene selene
 
-FROM rust:1.64-alpine3.14 AS selene-musl-builder
+FROM rust:${RUST_VERSION}-alpine AS selene-musl-builder
 RUN apk add g++ && \
     cargo install --branch main --git https://github.com/Kampfkarren/selene selene
 
-FROM rust:1.64-alpine3.14 AS selene-light-musl-builder
+FROM rust:${RUST_VERSION}-alpine AS selene-light-musl-builder
 RUN apk add g++ && \
     cargo install --no-default-features --branch main --git https://github.com/Kampfkarren/selene selene
 
@@ -32,4 +34,4 @@ CMD ["/selene"]
 
 FROM bash AS selene-light-musl
 COPY --from=selene-light-musl-builder /usr/local/cargo/bin/selene /
-CMD ["/selene"]
+CMD ["/selene"]