[bug] Extensions not copied for Docker runner #20281

Describe the bug

The Docker runner copies profiles, global.conf, settings.yml, remotes.json, and cache data into the container. It does not copy extensions.

This means extensions installed in the host Conan home are unavailable to Conan running inside the container.

In particular, the compatibility.py plugin doesn't get copied in.
So dependencies that would only be found with that don't work basically.

Observed with Conan 2.31.2.

How to reproduce it

No response