Use unique Docker volumes for isolated AppHost instances

### Is your feature request related to a problem? Please describe the problem.

Isolated mode randomizes ports and copies user secrets into an isolated store, but AppHost instances can still use the same Docker volume names. Running isolated instances concurrently, particularly from multiple git worktrees, can therefore share state or conflict through the same volumes.

### Describe the solution you'd like

Docker volumes used by an AppHost should have instance-unique names when running in isolated mode. Each isolated instance should receive independent volume state so it can run concurrently without affecting another checkout or instance. Explicitly external or intentionally shared volumes should retain a way to opt into sharing.

### Additional context

I searched the existing issues and did not find one covering Docker-volume isolation for isolated AppHost runs.

Start by tracing how AppHost isolated mode derives instance identity and how Docker volume names are assigned. Verify concurrent isolated runs use distinct names while explicitly external or intentionally shared volumes can still opt into sharing; the issue does not name files or tests.

Written by the indexing model from the issue text.