Add docker troubleshooting entry for npm ENOTEMPTY caused by macOS Docker VMM (beta)

[RayBB]

N/A — docs-only troubleshooting entry from a locally diagnosed & verified failure; no existing issue covers it.

Docs fix.

Technical

• Adds a Troubleshooting entry to docker/README.md for commands running npm inside containers failing consistently with npm error ENOTEMPTY: directory not empty, rmdir '/openlibrary/node_modules/...' (often with npm ls also reporting packages as UNMET DEPENDENCY despite the directories visibly existing).
• Root cause on macOS: Docker Desktop's Docker VMM (beta) virtual machine backend corrupting writes to named volumes (e.g. ol-nodemodules). The entry walks through the three-step fix: switch to the Apple Virtualization framework VM option, recreate affected *_ol-nodemodules volumes, re-run.
• Notes that other clones/worktrees' *_ol-nodemodules volumes populated while Docker VMM was active need the same treatment.

Testing

• Reproduced the failure deterministically with the Docker VMM (beta) backend (make css died on Makefile:20: node_modules, ENOTEMPTY on rmdir).
• After switching to Apple Virtualization framework (keeping VirtioFS): docker compose run --rm web make css completes successfully; npm ls --depth=0 reports 0 UNMET dependencies with the full 677-entry tree.

Screenshot

N/A — no UI change.

Stakeholders

Module: Docker