The gateway's config.yaml → env var bridge was missing docker_volumes,
so Docker volume mounts configured in config.yaml were ignored for
gateway sessions (Telegram, Discord, etc.) while working in CLI.
Also fixes list serialization: str() produces Python repr with single
quotes which json.loads() in terminal_tool.py can't parse. Now uses
json.dumps() for list values.
Based on PR #431 by @manuelschipper (applied manually due to stale branch).