fix(docker): bump rust:1.85 → 1.89 (matches workspace rust-toolchain.toml)

Build failed on the multi-arch run: `time@0.3.47 requires rustc 1.88.0`
and the workspace toolchain pin is already 1.89 (needed for ruvector-core's
avx512f target_feature, mmap-rs edition 2024, hnsw_rs is_multiple_of).
Dockerfile lagged on 1.85.

Refs #794.

Co-Authored-By: claude-flow <ruv@ruv.net>

docker/Dockerfile.rust

@@ -3,7 +3,7 @@
 # Multi-stage build for minimal final image
 
 # Stage 1: Build
-FROM rust:1.85-bookworm AS builder
+FROM rust:1.89-bookworm AS builder
 
 WORKDIR /build