Persist neovim plugin/Mason data across container recreations

Mason LSP installs and Lazy plugin cache live at ~/.local/share/nvim,
which was in the container's writable layer. Every --force-recreate
triggered a full re-download of all plugins and LSP servers on next
nvim launch — slow and wasteful.

Add devbox-nvim-data named volume in docker-compose.yml and
docker-compose.shared.yml, add to entrypoint ownership-fix loop,
update persistence tables in README.md and DOCKER_HUB.md.

entrypoint.sh

@@ -78,6 +78,7 @@ for dir in \
   /home/"$USER_NAME"/.local/state/opencode \
   /home/"$USER_NAME"/.local/share/uv \
   /home/"$USER_NAME"/.local/share/zoxide \
+  /home/"$USER_NAME"/.local/share/nvim \
   /home/"$USER_NAME"/.cache/bash \
   /home/"$USER_NAME"/.rustup \
   /home/"$USER_NAME"/.cargo \