chore(progress): document vehicle-routing Docker build blocker

The vehicle-routing Docker image build is blocked due to local path
dependencies that aren't available in the Docker build context:
- solverforge crate not yet published to crates.io
- solverforge-maps crate not yet published to crates.io

The code uses unreleased API features (shadow variables, list planning
variables) that don't exist in any published version. Task will be
unblocked when solverforge 0.6.0+ is published.

Author: blackopsrepl