Docker and `venv` environments should create cross-platform locks if possible

Can we use Docker/pip to solve the specs and lock for as many platforms as possible? This would eliminate the issue where moving to a new platform invalidates a bunch of stages. However, we may be then implying that the results are platform-independent, which may not be the case.

Start by tracing how Docker and Python venv environments currently solve specifications and create locks. Define which platforms can share locks without implying unsupported platform independence; done means the workflow avoids unnecessary invalidation while preserving correct platform-specific results.

Written by the indexing model from the issue text.