Use Docker if available (or option) for true isolation

Would be nice to use autochecker to use Docker for isolation and speed in parallel

https://github.com/VictorBjelkholm/autochecker

Start by reading the linked autochecker project, then inspect next-update's current test execution path to find where isolation and parallelism are handled. Define how Docker availability or an explicit option should affect that path; done means dependency checks run with true isolation and parallel execution without breaking environments where Docker is unavailable.

Written by the indexing model from the issue text.