Update Dockerfile for multi-stage Rust+Python build

[dmartmillan]

Description
Rewrite Dockerfile as a two-stage build: rust:slim builder stage compiles the wheel, python:slim final stage installs it.

Files

• Dockerfile — REWRITE

Definition of Done

• docker build -t openvariant . completes without error
• docker run openvariant openvar --help works
• Final image size ≤ Python base + ~10 MB (no Rust toolchain in final layer)

Documentation needed: Update README.md Docker usage section.

Tests needed: CI job: docker build on push to main.