Fix block_log symlinks for Docker container compatibility

Symlinks created for block_log files now point directly to the source
path (e.g., /blockchain/block_log_5m/...) instead of the intermediate
path (/cache/../blockchain/...). This ensures symlinks work inside
Docker containers.

Additionally, build_data.sh now mounts the block_log source directory
read-only when running the container, so the symlinks can be resolved.