Modify containers to point VS Code extension at manually installed Dafny

Instead of ending up with two copies that are in danger of being different versions by accident

Inspect the repository's container definitions and VS Code extension setup first; the issue does not name specific files or entry points. Trace where Dafny is installed and how the extension locates it, then verify that the container uses the manually installed version without creating a second copy.

Written by the indexing model from the issue text.