httpd: Dockerfile uses Rust version from rust-toolchain

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.

Co-authored-by: Fintan Halpenny <fintan.halpenny@gmail.com>

radicle-httpd/build/Dockerfile

@@ -1,5 +1,7 @@
 # Builds release binaries for Radicle.
-FROM rust:1.83.0-alpine3.21 as builder
+ARG RUST_VERSION="1.87"
+ARG ALPINE_VERSION="3.20"
+FROM rust:${RUST_VERSION}-alpine${ALPINE_VERSION} as builder
 LABEL maintainer="Radicle Team <team@radicle.xyz>"
 WORKDIR /src
 COPY . .

radicle-httpd/build/build

@@ -34,6 +34,7 @@ main() {
   keypath="$(rad path)/keys/radicle.pub"
   version="$(build/version)"
   image=radicle-build-$version
+  rust_version="$(cat ./rust-toolchain)"
 
   if [ ! -f "$keypath" ]; then
     echo "fatal: no key found at $keypath" >&2
@@ -51,6 +52,7 @@ main() {
   case "$OS" in
   Darwin)
     podman build \
+      --build-arg "RUST_VERSION=$rust_version" \
       --build-arg SOURCE_DATE_EPOCH \
       --build-arg TZ \
       --build-arg LC_ALL \
@@ -63,6 +65,7 @@ main() {
     ;;
   *)
     podman --cgroup-manager=cgroupfs build \
+      --build-arg "RUST_VERSION=$rust_version" \
       --build-arg SOURCE_DATE_EPOCH \
       --build-arg TZ \
       --build-arg LC_ALL \