fix: handle volume permissions correctly

[tiero]

Fix volume permissions for Linux users

This PR fixes permission issues with mounted volumes, particularly on Linux systems where users were experiencing root ownership problems.

Testing on macOS

# 1. Make sure Docker Desktop is running
# 2. Build and test
make build

# 3. Run test cycle with the new binary
sudo rm -rf ~/Library/Application\ Support/Nigiri
./build/nigiri-darwin-arm64 stop --delete
./build/nigiri-darwin-arm64 start

# 4. Verify permissions (should show your user, not root)
ls -la ~/Library/Application\ Support/Nigiri/volumes/bitcoin/

Expected output should show your user owning the files:

drwxr-x---   4 <your-user>  staff  128 Dec 18 19:19 .
drwxr-x---   7 <your-user>  staff  224 Dec 18 19:19 ..
-rw-r-----   1 <your-user>  staff  295 Dec 18 19:19 bitcoin.conf
drwx------  12 <your-user>  staff  384 Dec 18 19:19 regtest

[tiero]

I managed to test on a debian VM. Now also Bitcoin Core datadir is owned by the user, and not root!