fix(docker): bump Rust toolchain from 1.82 to 1.85 for moxcms compatibility

moxcms-0.8.1 (transitive dependency via image-0.25) uses edition 2024
and rust-version 1.85.0, which cannot be parsed by Rust 1.82.
This caused the Docker build to fail with:
  error: failed to parse manifest at moxcms-0.8.1/Cargo.toml

Upgrading the build stage from rust:1.82-bookworm to rust:1.85-bookworm
resolves the Cargo.toml parsing error.

Dockerfile

@@ -8,7 +8,7 @@
 # ---------------------------------------------------------------------------
 # Stage 1: Rust build
 # ---------------------------------------------------------------------------
-FROM rust:1.82-bookworm AS rust-builder
+FROM rust:1.85-bookworm AS rust-builder
 
 RUN apt-get update && apt-get install -y \
     pkg-config \