build: Dockerfile uses Rust version from `rust-toolchain.toml`

Instead of hardcoding the version of the Rust alpine file, introduce an `ARG` in
the `Dockerfile`. In `build/build`, we determine the version by inspecting
`rust-toolchain.toml`, and pass this value in. This ensures that the binaries
are built using the same toolchain that we develop with.

build/Dockerfile

@@ -1,5 +1,8 @@
 # Builds release binaries for Radicle.
-FROM rust:1.80-alpine3.20 as builder
+ARG RUST_VERSION="1.85"
+ARG ALPINE_VERSION="3.20"
+
+FROM rust:${RUST_VERSION}-alpine${ALPINE_VERSION} as builder
 LABEL maintainer="Radicle Team <team@radicle.xyz>"
 WORKDIR /src
 COPY . .

build/build

@@ -27,6 +27,7 @@ main() {
   keypath="$(rad path)/keys/radicle.pub"
   version="$(build/version)"
   image=radicle-build-$version
+  rust_version="$(build/rust-version)"
 
   if [ ! -f "$keypath" ]; then
     echo "fatal: no key found at $keypath" >&2 ; exit 1
@@ -40,6 +41,7 @@ main() {
 
   echo "Building image ($image).."
   podman --cgroup-manager=cgroupfs build \
+    --build-arg "RUST_VERSION=$rust_version" \
     --build-arg SOURCE_DATE_EPOCH \
     --build-arg TZ \
     --build-arg LC_ALL \

build/rust-version

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+if ! version=$(grep -m 1 -oP "channel\s*=\s*\K(.*)" rust-toolchain.toml); then
+  echo "fatal: no rust version found via rust-toolchain.toml" >&2 ; exit 1
+fi
+
+echo "$version"